Equity Method Investments and Joint Ventures [Abstract] Real Estate Joint Ventures Real Estate Joint Ventures Disclosure [Text Block] This block of text may be used to disclose details concerning consolidated and unconsolidated real estate joint ventures. Accounting Policies [Abstract] Summary of Significant Accounting Policies Basis of Presentation and Significant Accounting Policies [Text Block] Equity [Abstract] Equity Stockholders' Equity Note Disclosure [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Schedule of Real Estate [Table] Schedule of Real Estate [Table] Schedule that includes carrying amount of properties, accumulated depreciation and other information for real estate properties, by status. Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] GI-COPT GI-COPT DC Partnership LLC [Member] GI-COPT DC Partnership LLC [Member] Real Estate Properties Status [Axis] Real Estate Properties Status [Axis] Information by status of the property. Real Estate Properties Status [Domain] Real Estate Properties Status [Domain] Represents the status of the property. Operating Properties Operating Properties [Member] Represents information pertaining to operating properties of the entity. Properties under, or contractually committed for, construction or approved for redevelopment Properties Under Construction, Redevelopment or Contractually Committed for Construction [Member] Represents properties under, or contractually committed for, construction or approved for redevelopment. Land controlled for future development Land Controlled for Future Development [Member] Represents land controlled by the entity for future development. Other Land Other Land [Member] Other Land [Member] Operating wholesale data centers Wholesale Data Center [Member] Represents the wholesale data center segment of the entity. Real Estate Properties [Axis] Real Estate Property Ownership [Axis] Real Estate Properties [Domain] Real Estate Properties [Domain] Office Properties Office Properties [Member] Office Properties [Member] Single-tenant data centers Single-tenant Data Centers [Member] Represents single-tenant data centers owned by the entity. Partially operational properties Partially Operational Properties [Member] Represents office properties under construction or redevelopment which are owned by the entity and which are partially operational. Investments in real estate Real Estate Properties [Line Items] Number of real estate properties Number of Real Estate Properties Area of real estate property (in square feet or acres) Area of Real Estate Property Critical load (in megawatts) Critical Load Represents the amount of the critical load of the property. Critical load is the amount of megawatts of power made available for the exclusive use tenants in the property. Receivables [Abstract] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Receivable Type [Axis] Receivable Type [Axis] Receivable [Domain] Receivable [Domain] Notes Receivable from City of Huntsville Notes Receivable from City of Huntsville [Member] Represents notes receivable from City of Huntsville. Other investing loans receivable Loans and Finance Receivables [Member] LW Redstone Company, LLC LW Redstone Company LLC [Member] Represents the investment in L W Redstone Company, LLC, a consolidated joint venture. Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Investing receivables Financing Receivable, Net Stated interest rate (as a percent) Note Receivable, Terms, Interest Rate Note Receivable, Terms, Interest Rate Fair Value Disclosures [Abstract] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value by Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair value measurement on a recurring basis Fair Value, Measurements, Recurring [Member] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] Corporate Office Properties, L.P. Subsidiaries [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Investment Type [Axis] Investment Type [Axis] Investment Type Categorization [Domain] Investments [Domain] Mutual funds Equity Funds [Member] Other Other Marketable Securities [Member] Represents other marketable securities not otherwise specified in taxonomy. Assets and liabilities measured at fair value on a recurring basis Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Assets: Assets, Fair Value Disclosure [Abstract] Marketable securities in deferred compensation plan Deferred Compensation Plan Assets Interest rate derivatives Interest Rate Derivative Assets, at Fair Value Total assets Assets, Fair Value Disclosure Liabilities: Liabilities, Fair Value Disclosure [Abstract] Deferred compensation plan liability Deferred Compensation Liability, Current and Noncurrent Earnings Per Share [Abstract] Schedule of Subsidiary or Equity Method Investee [Table] Schedule of Subsidiary or Equity Method Investee [Table] Subsidiary or Equity Method Investee [Line Items] Subsidiary or Equity Method Investee [Line Items] Numerator: Net Income (Loss) Available to Common Stockholders, Diluted [Abstract] Net income attributable to COPT Net Income (Loss) Attributable to Parent Preferred share/unit dividends/distributions Preferred Stock Dividends, Income Statement Impact Income attributable to share-based compensation awards Participating Securities, Distributed and Undistributed Earnings (Loss), Basic Numerator for basic and diluted EPS on net income attributable to COPT common shareholders Net Income (Loss) Attributable to Common Stockholders, Basic and Diluted Represents the net income after adjustments for gain on sales of real estate, preferred share dividends, income attributable to noncontrolling interests and income from attributable to restricted shares used to calculate the basic and dilutive earnings per share. Denominator (all weighted averages): Weighted Average Number of Shares Outstanding, Diluted [Abstract] Denominator for basic EPS (common shares) Weighted Average Number of Shares Outstanding, Basic Dilutive effect of share-based compensation awards (shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Denominator for diluted EPS (common shares) Weighted Average Number of Shares Outstanding, Diluted Basic EPS: Earnings Per Share, Basic [Abstract] Net income attributable to COPT common shareholders - basic (in dollars per share/unit) Earnings Per Share, Basic Diluted EPS: Earnings Per Share, Diluted [Abstract] Net income attributable to COPT common shareholders - diluted (in dollars per share/unit) Earnings Per Share, Diluted Segment Reporting [Abstract] Reconciliation of segment revenues to total revenues Segment Reporting Revenue Reconciling Item [Abstract] Segment revenues from real estate operations Real Estate Revenue, Net Construction contract and other service revenues Contracts Revenue Total revenues Revenues Reconciliation of UJV NOI allocable to COPT to Equity Income in Unconsolidated Entities Reconciliation of Operating Income from Unconsolidated Joint Ventures to Equity Method Investments [Abstract] Reconciliation of Operating Income from Unconsolidated Joint Ventures to Equity Method Investments [Abstract] UJV NOI allocable to COPT Unconsolidated Joint Venture Net Operating Income Allocable to Parent Unconsolidated Joint Venture Net Operating Income Allocable to Parent Less: Income from UJV allocable to COPT attributable to depreciation and amortization expense and interest expense Income (Loss) from Equity Method Investments Allocable to Depreciation, Amortization and Interest Expense Income (Loss) from Equity Method Investments Allocable to Depreciation, Amortization and Interest Expense Add: Equity in loss of unconsolidated non-real estate entities Income (Loss) in Non-Real Estate Equity Method Investments Income (Loss) in Non-Real Estate Equity Method Investments Equity in income of unconsolidated entities Income (Loss) from Equity Method Investments Computation of net operating income from service operations Operating Income (Loss) [Abstract] Construction contract and other service revenues Construction contract and other service expenses Construction Contract and Other Service Expenses The aggregate costs related to construction and development services and the aggregate costs related to other service operations during the reporting period NOI from service operations Net Operating Income (Loss) from Service Operations Represents the computation of NOI from service operations, which is the difference between Construction contract and other service revenues and Construction contract and other service expenses. New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Cumulative Distributions in Excess of Net Income Accumulated Distributions in Excess of Net Income [Member] Common Shares Common Stock [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2017-05 Accounting Standards Update 2017-05 [Member] Accounting Standards Update 2017-05 [Member] Accounting Standards Update 2017-12 Accounting Standards Update 2017-12 [Member] Accounting Standards Update 2017-12 [Member] Lease Arrangement, Type [Axis] Lease Arrangement, Type [Axis] Lease Arrangement, Type [Domain] Lease Arrangement, Type [Domain] Ground Leases Ground Leases [Member] Ground Leases [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Effect of change on net income New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income Cumulative effect of accounting change for adoption of hedge accounting guidance Cumulative Effect of New Accounting Principle in Period of Adoption Minimum rental payments due Operating Leases, Future Minimum Payments Due Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] 2018 PSU Grants Two Thousand Eighteen PSU Grants [Member] Two Thousand Eighteen PSU Grants [Member] 2015 PSU Grants Two Thousand Fifteen PSU Grants [Member] Two Thousand Fifteen PSU Grants [Member] Award Type [Axis] Award Type [Axis] Award Type [Domain] Equity Award [Domain] Performance share units Performance Shares [Member] Restricted shares Restricted Stock [Member] Title of Individual [Axis] Title of Individual [Axis] Relationship to Entity [Domain] Relationship to Entity [Domain] Executives Officer [Member] Share-Based Compensation Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Stock awards granted (in shares or units) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Aggregate grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Aggregate Fair Value The aggregate fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). Number of employees affected Deferred Compensation Arrangement with Individual, Number of Individuals Deferred Compensation Arrangement with Individual, Number of Individuals Potential earned PSUs payout for defined levels of performance under awards Share-based Compensation Arrangement by Share-based Payment Award, Earned Performance Share Units, Payout Percentage [Abstract] Earned PSUs payout (as a percent of PSUs granted) on 75th or greater percentile rank Share-based Compensation Arrangement by Share-based Payment Award of Earned, Performance Share Units, Payout Percentage Based on Percentile Rank Greater than or Equals to 75 Represents the earned performance share units payout, as a percentage of performance share units granted, if the entity's total shareholder return relative to a peer group of companies ranks at the 75th percentile or greater. Earned PSUs payout (as a percent of PSUs granted) on 50th percentile rank Share-based Compensation Arrangement by Share-based Payment Award of Earned, Performance Share Units, Payout Percentage Based on Percentile Rank Equals to 50 Represents the earned performance share units payout, as a percentage of performance share units granted, if the entity's total shareholder return relative to a peer group of companies ranks at the 50th percentile. Earned PSUs payout (as a percent of PSUs granted) on 25th percentile rank Share-based Compensation Arrangement by Share-based Payment Award of Earned, Performance Share Units, Payout Percentage Based on Percentile Rank Equals to 25 Represents the earned performance share units payout, as a percentage of performance share units granted, if the entity's total shareholder return relative to a peer group of companies ranks at the 25th percentile. Performance share units granted on percentile rank below 25th (as a percent) Share-based Compensation Arrangement by Share-based Payment Award of Earned, Performance Share Units, Payout Percentage Based on Percentile Rank Less than 25 Represents the earned performance share units payout percentage based on percentile rank below 25. The number of percentile ranks to fall between to earn interpolated PSUs between such percentile ranks, conditioned on the percentile rank exceeding 25% Number of Percentile Ranks to Fall Between to Earn Interpolated PSU The number of percentile ranks to fall between to earn interpolated PSUs between such percentile ranks, conditioned on the percentile rank exceeding 25%. Assumptions used to value stock awards Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] Performance period of the award Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period Weighted average grant date fair value (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Baseline value per common share (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Baseline Common Share Value Represents the baseline common share value that is used in valuing the grant date fair value of awards. Expected volatility of common shares (as a percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Risk-free interest rate (as a percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Other Share-based Compensation Additional Disclosures Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract] Shares issued (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Shares issued for Awards Vested in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Shares issued for Awards Vested in Period Percentage of target Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Percentage of Target Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Percentage of Target Shares vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Weighted average fair value of shares vested (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Aggregate intrinsic value of awards upon vesting Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested Schedule of New Accounting Pronouncements and Changes in Accounting Principles Schedule of New Accounting Pronouncements and Changes in Accounting Principles [Table Text Block] Revenue from Contract with Customer [Abstract] Revenue Recognition on Construction Contract and Other Service Revenues Revenue from Contract with Customer [Text Block] Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Partner Type [Axis] Partner Type [Axis] Partner Type of Partners' Capital Account, Name [Domain] Partner Type of Partners' Capital Account, Name [Domain] Limited Partner Limited Partner [Member] General Partner General Partner [Member] Preferred Shares Preferred Stock [Member] Additional Paid-in Capital Additional Paid-in Capital [Member] Noncontrolling Interests Noncontrolling Interest [Member] Sale of Stock [Axis] Sale of Stock [Axis] Sale of Stock [Domain] Sale of Stock [Domain] Common Stock Issued to Public Under At-the-Market Program Common Stock At-the-Market Program [Member] Common Stock At-the-Market Program [Member] Forward Equity Sale Agreement Forward Equity Sale Agreement [Member] Forward Equity Sale Agreement [Member] Statement [Line Items] Statement [Line Items] Increase (Decrease) in Shareholders' Equity Increase (Decrease) in Stockholders' Equity [Roll Forward] Balance (COPT: 98,498,651 and 101,292,299 shares as of December 31, 2016 and 2017, respectively) Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Balance (preferred units) Preferred Units, Outstanding Balance (in units/shares) Common Stock, Shares, Outstanding Balance, as adjusted Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance Conversion of common units to common shares (COPT: 185,000 and 53,817 shares as for the three months ended March 31, 2017 and 2018, respectively) Stock Issued During Period, Value, Conversion of Units Common shares issued during the period Stock Issued During Period, Value, New Issues Common shares issued during the period (in shares) Stock Issued During Period, Shares, New Issues Share-based compensation issuance, net of redemptions (in units/shares) Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures Share-based compensation issuance, net of redemptions (COPT: 159,801 and 127,242 shares issued, net of redemptions for the three months ended March 31, 2017 and 2018, respectively) Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Redemption of vested equity awards Stock Repurchased During Period, Value Adjustments to noncontrolling interests resulting from changes in ownership of COPLP Noncontrolling Interest, Increase from Subsidiary Equity Issuance Comprehensive income Comprehensive Income (Loss), Net of Tax, After Redeemable Noncontrolling Interest Adjustments Comprehensive Income (Loss), Net of Tax, After Redeemable Noncontrolling Interest Adjustments Dividends / Distributions to owners of common and preferred units Dividends Distributions to owners of common and preferred units in COPLP Minority Interest (Decrease) from Distributions to Operating Partnership Noncontrolling Interest Holders Decrease in noncontrolling interest balance from payment of distributions to noncontrolling interest holders in the Operating Partnership. Distributions to noncontrolling interests in subsidiaries Minority Interest (Decrease) from Distributions Including Joint Ventures Decrease in noncontrolling interest balance as a result of distributions to equity interests owned by noncontrolling shareholders, partners, or other equity holders in joint ventures included in the entity's consolidated financial statements. Adjustment to arrive at fair value of redeemable noncontrolling interests Adjustment to Equity to Arrive at Fair Value of Redeemable Noncontrolling Interest Adjustment to equity to arrive at fair value of redeemable noncontrolling interest Balance (in units/shares) Balance (preferred units) Balance (COPT: 99,390,234 shares and 102,150,358 shares as of March 31, 2017 and 2018, respectively) Schedule of payouts for defined performance under performance-based awards of share-based compensation Schedule of Share-based Compensation Arrangement by Share-based Payment Award for Defined Performance [Table Text Block] Schedule of payouts for defined performance under performance-based awards of share-based compensation Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Fixed rate debt Fixed Rate Secured Debt [Member] A collateralized debt obligation (backed by pledge, mortgage or other lien on the entity's assets) bearing fixed interest rates. Variable-rate debt Variable Rate Debt [Member] Variable Rate Debt Revolving Credit Facility Revolving Credit Facility [Member] Term Loan Facilities Term Loan Facilities [Member] Represents unsecured term loan agreements entered into by entity. 3.60% Senior Notes 3.60% Senior Notes [Member] 3.60% Senior Notes [Member] 5.250% Senior Notes 5.250% Senior Notes [Member] 5.250% Senior Notes [Member] 3.70% Senior Notes 3.70% Senior Notes [Member] 3.70SeniorNotes [Member] 5.000% Senior Notes Five Percent Senior Notes [Member] Five Percent Senior Notes [Member] Unsecured notes payable Notes Payable, Other Payables [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Secured Debt Secured Debt [Member] Line of Credit Line of Credit [Member] Loans payable Loans Payable [Member] Unsecured senior notes Senior Notes [Member] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) [Member] Debt Debt Instrument [Line Items] Carrying Value Long-term Debt Debt instrument, face amount Debt Instrument, Face Amount Variable rate, spread (as a percent) Debt Instrument, Basis Spread on Variable Rate Deferred financing costs, net Debt Issuance Costs, Net Unamortized premium included in carrying value Debt Instrument, Unamortized Premium Weighted average interest rate (as a percent) Debt, Weighted Average Interest Rate Stated interest rate (as a percent) Debt Instrument, Interest Rate, Stated Percentage Number of extensions Line of Credit Facility, Maturity Date Extension, Number of Extensions Line of Credit Facility, Maturity Date Extension, Number of Extensions Extension option period (in years) Debt Instrument, Extension Option, Term Represents the period over which the debt may be extended at the option of the entity, subject to certain conditions. Line of credit facility, extension fee percentage Line of Credit Facility, Extension Fee Percentage Line of Credit Facility, Extension Fee Percentage Aggregate additional borrowing capacity available Credit Facility, Aggregate Additional Borrowing Capacity Available Credit Facility, Aggregate Additional Borrowing Capacity Available Unamortized discount included in carrying value Debt Instrument, Unamortized Discount Interest rate on debt (as a percent) Debt Instrument, Interest Rate, Effective Percentage Interest costs capitalized Interest Costs Capitalized Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative [Table] Derivative [Table] Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Designated Designated as Hedging Instrument [Member] Derivative Instrument Risk [Axis] Derivative Instrument [Axis] Derivative Contract Type [Domain] Derivative Contract [Domain] Interest rate swaps Interest Rate Swap [Member] Interest rate swap, effective date September 1, 2015, swap two Interest Rate Swap, Effective Date September 1, 2015 Two [Member] Interest Rate Swap, Effective Date September 1, 2015 Two [Member] Interest rate swap, effective October 13, 2015 Interest Rate Swap, Effective October 13, 2015 [Member] Interest Rate Swap, Effective October 13, 2015 [Member] Interest rate swap, effective September 1, 2016, swap one Interest Rate Swap, Effective September 1, 2016, One [Member] Interest Rate Swap, Effective September 1, 2016, One [Member] Interest rate swap, effective September 1, 2016, swap two Interest Rate Swap, Effective September 1, 2016, Two [Member] Interest Rate Swap, Effective September 1, 2016, Two [Member] Interest rate swap, effective September 1, 2016, swap three Interest Rate Swap, Effective September 1, 2016, Three [Member] Interest Rate Swap, Effective September 1, 2016, Three [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Prepaid expenses and other current assets Prepaid Expenses and Other Current Assets [Member] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Interest Expense Interest Expense [Member] Fair values of interest rate swap derivatives Derivative [Line Items] Notional Amount Derivative, Notional Amount Fixed rate (as a percent) Derivative, Fixed Interest Rate Fair value of interest rate swaps Interest Rate Derivatives, at Fair Value, Net Notional amount of interest rate derivatives after scheduled amortization Interest Rate Cash Flow Hedge, Derivative Notional Amount after Scheduled Amortization Represents the notional amount of interest rate derivatives after scheduled amortization. Fair value of interest rate derivatives and balance sheet classification Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract] Interest rate swaps designated as cash flow hedges Interest Rate Cash Flow Hedge Asset at Fair Value Effect of interest rate derivatives on consolidated statements of operations and comprehensive income Summary of Derivative Instruments Impact on Results of Operations [Abstract] Interest rate derivatives Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax Amount of Gain (Loss) Reclassified from AOCI into Income Derivative Instruments, Loss Reclassified from Accumulated OCI into Income, Effective Portion Approximate loss amount to be reclassified from AOCI to interest expense over the next 12 months Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net Noncontrolling Interest [Abstract] Redeemable Noncontrolling Interests Noncontrolling Interest Disclosure [Text Block] Earnings Per Share (“EPS”) and Earnings Per Unit (“EPU”) Earnings Per Share [Text Block] Adjustments for Change in Accounting Principle [Axis] Adjustments for Change in Accounting Principle [Axis] Adjustments for Change in Accounting Principle [Domain] Adjustments for Change in Accounting Principle [Domain] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] As Previously Reported Scenario, Previously Reported [Member] Investment in unconsolidated real estate joint venture Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures Cumulative distributions in excess of net income Accumulated Distributions in Excess of Net Income Noncontrolling interests in subsidiaries Stockholders' Equity Attributable to Noncontrolling Interest Common units Common Stock, Value, Issued Equity in income of unconsolidated entities Income before gain on sales of real estate Income (Loss) before Gain (Loss) on Sale of Properties Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net income attributable to noncontrolling interests - Common units in COPLP Noncontrolling Interest in Net Income (Loss) Operating Partnerships, Nonredeemable Net income attributable to COPT common shareholders Net Income (Loss) Available to Common Stockholders, Basic Comprehensive income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Comprehensive income attributable to COPT Comprehensive Income (Loss), Net of Tax, Attributable to Parent Number of joint ventures with redeemable noncontrolling interests Number of Joint Ventures With Redeemable Noncontrolling Interests Number of Joint Ventures With Redeemable Noncontrolling Interests Redeemable Noncontrolling Interest [Roll Forward] Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward] Beginning balance Redeemable Noncontrolling Interest, Equity, Fair Value Distributions to noncontrolling interests Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Net income attributable to noncontrolling interests Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Adjustment to arrive at fair value of interests Noncontrolling Interest, Change in Redemption Value Ending balance Deferred Bonus and Profit Sharing Arrangements, Individual Contracts, Type of Deferred Compensation [Axis] Deferred Bonus and Profit Sharing Arrangements, Individual Contracts, Type of Deferred Compensation [Axis] Deferred Bonus and Profit Sharing Arrangement, Individual Contract, Type of Deferred Compensation [Domain] Deferred Bonus and Profit Sharing Arrangement, Individual Contract, Type of Deferred Compensation [Domain] Deferred compensation plan Deferred Compensation, Excluding Share-based Payments and Retirement Benefits [Member] Management Management [Member] Assets and liabilities measured at fair value Maximum percentage of participants' compensation which is deferrable (as a percent) Deferred Compensation Arrangement with Individual Maximum Deferrable Percentage of Compensation Represents the maximum percentage of participants' compensation that is deferrable by participants under the deferred compensation plan. Marketable securities in deferred compensation plan Schedule of debt Schedule of Debt [Table Text Block] Schedule of the fair value of debt Schedule of Fair Value of Debt [Table Text Block] Tabular disclosure of carrying value and fair value of debt, by debt type. Schedule of Limited Liability Company LLC or Limited Partnership LP Managing Member or General Partner Ownership Interest [Table] Schedule of Limited Liability Company LLC or Limited Partnership LP Managing Member or General Partner Ownership Interest [Table] Disclosures related to the ownership of the LLC or LP by the managing member or general partner. Units by Name [Axis] Units by Name [Axis] The axis of a table defines the relationship between the domain members or categories in the table and the line items or concepts that complete the table. Units, Class [Domain] Units, Class [Domain] Type or class of preferred units or special units. Common Units Common Units [Member] Common units representing ownership interest in the operating partnership. Forms of ownership in Operating Partnership and ownership percentage by the entity Ownership in Subsidiary Percentage [Line Items] Line items related to the percentage of classes of ownership in a subsidiary that are owned by the parent. Percentage ownership in operating partnership Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest Interest Rate Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping, Disclosure Item Amounts [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Carrying Amount Reported Value Measurement [Member] Estimated Fair Value Estimate of Fair Value Measurement [Member] Other fixed-rate debt Other Fixed Rate Debt [Member] Fixed-rate debt excluding other fixed rate debt not separately disclosed. Carrying amount and estimated fair value of debt Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-term debt Long-term Debt, Fair Value Accounting Standards Update 2016-18 Accounting Standards Update 2016-18 [Member] Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Net cash provided by investing activities Net Cash Provided by (Used in) Investing Activities Net increase in cash and cash equivalents and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Beginning of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents End of period Real Estate [Abstract] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Real Estate Property Disposal [Axis] Real Estate Property Disposal [Axis] Represents information regarding disposal of operating properties aggregated according to factors such as location, number of properties and acquisition date. Real Estate Property Disposal [Domain] Real Estate Property Disposal [Domain] Represents operating property disposal. 11751 Meadowville Lane Number 11751 Meadowville Lane [Member] Represents the information pertaining to 11751 Meadowville Lane, an office property owned by the entity. Real Estate Property Classification [Axis] Real Estate Property Classification [Axis] Represents information regarding classification of real estate properties. Real Estate Property Classification [Domain] Real Estate Property Classification [Domain] Represents classification of real estate properties. Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Defense/Information Technology Locations Defense and Technology Locations [Member] Defense and Technology Locations [Member] Subsegments [Axis] Subsegments [Axis] Subsegments [Domain] Subsegments [Domain] Data Center Shells Data Center Shells [Member] Data Center Shells [Member] Disposal Group Classification [Axis] Disposal Group Classification [Axis] Disposal Group Classification [Domain] Disposal Group Classification [Domain] Assets held for sale, net Disposal Group, Held-for-sale, Not Discontinued Operations [Member] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Properties, net Disposal Group, Including Discontinued Operation, Properties, Net Disposal Group, Including Discontinued Operation, Properties, Net Deferred rent receivable Disposal Group, Including Discontinued Operation, Deferred Rent Receivable Disposal Group, Including Discontinued Operation, Deferred Rent Receivable Deferred leasing costs, net Disposal Group, Including Discontinued Operation, Deferred Leasing Costs, Net Disposal Group, Including Discontinued Operation, Deferred Leasing Costs, Net Assets held for sale, net Disposal Group, Including Discontinued Operation, Assets Indemnification amount Guarantor Obligations, Maximum Exposure, Undiscounted Information by Business Segment Segment Reporting Disclosure [Text Block] Real Estate, Joint Ventures [Table] Real Estate, Joint Ventures [Table] Tabular disclosure of information related to unconsolidated and consolidated real estate joint ventures. Operating office properties Operating Office Properties [Member] Represents operating office properties owned by the entity. Variable Interest Entities [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Consolidated real estate joint ventures Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] Variable Interest Entity, Primary Beneficiary Variable Interest Entity, Primary Beneficiary [Member] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] M Square Associates, LLC M Square Associates LLC [Member] Represents the investment in M Square Associates, LLC, a consolidated joint venture. Stevens Investors, LLC Stevens Investors, LLC [Member] Stevens Investors, LLC [Member] Investments in real estate joint ventures Real Estate Joint Venture [Line Items] Investments in consolidated real estate joint ventures Consolidated Joint Ventures Disclosure [Abstract] Ownership (as a percent) Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Total Assets Variable Interest Entity, Consolidated, Carrying Amount, Assets Encumbered Assets Pledged Assets, Not Separately Reported, Real Estate Total Liabilities Variable Interest Entity, Consolidated, Carrying Amount, Liabilities Ownership percentage (percent) Equity Method Investment, Ownership Percentage Real estate equity method investments Equity Method Investments Schedule of investing receivables Schedule of Mortgage and Other Investing Receivables [Table Text Block] Tabular disclosure of the carrying amounts of mortgage and other investing receivables, by major class. Reconciliation of NOI from real estate operations and NOI from service operations to (loss) income from continuing operations Segment Reporting Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Abstract] NOI from real estate operations Segment Reporting Operating Income Includes real estate revenues and property operating expenses from continuing and discontinued operations; and the net of revenues and property operating expenses of real estate operations owned through unconsolidated joint ventures allocable to parent. NOI from service operations Interest and other income Other Nonoperating Income Equity in income of unconsolidated entities Income tax expense Income Tax Expense (Benefit) Depreciation and other amortization associated with real estate operations Depreciation and Amortization for Real Estate Operations The current period expense charged against earnings on long-lived, physical real estate operations assets, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset or deferred costs associated with real estate operations over the benefit period of such asset or costs; or to reflect consumption during the period of an asset that is not used in production. General, administrative and leasing expenses Selling, General and Administrative Expense Business development expenses and land carry costs Business Development Expenses and Land Carry Costs Costs incurred for business development, which involves the development of products and services, their delivery, design and their implementation. Business development includes a number of techniques designed to grow an economic enterprise. It includes the non capitalized cost of evaluating and executing acquisitions. It also includes expenses incurred in connection with properties not in operations. Interest expense Interest and Debt Expense Less: UJV NOI allocable to COPT included in equity in income of unconsolidated entities Projects in development or held for future development Projects in Development or held-for-future Development [Member] Represents projects in development or held for future development. Property, Plant and Equipment by Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Land in development or held for future development Land in Development or held-for-future Development [Member] Represents land in development or held for future development. Construction in progress, excluding land Construction in Progress [Member] Properties Property, Plant and Equipment [Line Items] Projects in development or held for future development Development in Process Share-Based Compensation Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Statement of Cash Flows [Abstract] Cash flows from operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] Revenues from real estate operations received Revenues from Real Estate Operations Received Cash received for rents, tenant recoveries and other real estate operations during the current period. Construction contract and other service revenues received Construction Contract and Other Service Revenues Received Cash received for building and real estate development, which may include improvements (such as drainage, utilities, grading and subdividing) to a parcel of land, and general contracting services and from the sale of other goods or rendering of other services. Property operating expenses paid Property Operating Expenses Paid This element represents the amount of cash paid during the current period for costs that are directly related to generation of revenues from leased and rented properties or equipment. Construction contract and other service expenses paid Construction Contract and Other Service Expenses Paid Cash payments for costs related to construction and development services and other service operations during the current period. General, administrative, leasing, business development and land carry costs paid General and Administrative, Leasing, Business Development and Land Carry Paid Cash payments for aggregate total costs related to selling a firm's product and services, all other general and administrative expenses, business development and land carry costs in connection with properties not in operations. Interest expense paid Interest Paid, Net Lease incentives paid Payments of Lease Incentives Payments of Lease Incentives Other Other Operating Activities, Cash Flow Statement Net cash provided by operating activities Cash flows from investing activities Net Cash Provided by (Used in) Investing Activities [Abstract] Construction, development and redevelopment Payments to Develop Real Estate Assets Tenant improvements on operating properties Payments for Tenant Improvements Other capital improvements on operating properties Payments for Capital Improvements Proceeds from dispositions of properties Proceeds from Sale of Real Estate Held-for-investment Leasing costs paid Payments for Leasing Costs Other Payments for (Proceeds from) Other Investing Activities Net cash (used in) provided by investing activities Cash flows from financing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from debt Proceeds from Issuance of Long-term Debt [Abstract] Revolving Credit Facility Proceeds from Long-term Lines of Credit Repayments of debt Repayments of Long-term Debt [Abstract] Revolving Credit Facility Repayments of Long-term Lines of Credit Scheduled principal amortization Scheduled Principal Amortization Repayments of Long-term Debt The cash outflow for scheduled principal amortization payments made on debt initially having maturity due after one year or beyond the normal operating cycle, if longer. Payments on capital lease obligation Repayments of Long-term Capital Lease Obligations Net proceeds from issuance of common shares/units Proceeds from Issuance of Common Stock Redemption of preferred shares/units Payments for Repurchase of Redeemable Preferred Stock Common share/unit dividends/distributions paid Payments of Ordinary Dividends, Common Stock Preferred share/unit dividends/distributions paid Payments of Ordinary Dividends, Preferred Stock and Preference Stock Distributions paid to noncontrolling interests in COPLP Payments of Ordinary Dividends, Noncontrolling Interest Redemption of vested equity awards Payments for Repurchase of Common Stock Other Proceeds from (Payments for) Other Financing Activities Net cash provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities Net (decrease) increase in cash and cash equivalents and restricted cash Cash and cash equivalents and restricted cash Cash and Cash Equivalents, at Carrying Value [Abstract] Reconciliation of net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Net income Adjustments to reconcile net income to net cash provided by operating activities: Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Depreciation, Depletion and Amortization Amortization of deferred financing costs and net debt discounts Amortization of Debt Issuance Costs and Discounts Increase in deferred rent receivable Straight Line Rent Gain on sales of real estate Gain (Loss) on Sale of Properties Share-based compensation Share-based Compensation Other Other Noncash Income (Expense) Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Decrease in accounts receivable Increase (Decrease) in Accounts Receivable Decrease in prepaid expenses and other assets, net Increase (Decrease) in Prepaid Expense and Other Assets Decrease in accounts payable, accrued expenses and other liabilities Increase (Decrease) in Operating Liabilities Increase (decrease) in rents received in advance and security deposits Increase (Decrease) in Advance Rent and Security Deposit Liability The net change in amount that has been received by the entity that represents collections of cash or other assets related to real estate operations revenue for which revenue has not yet been recognized and security deposits. Reconciliation of cash and cash equivalents and restricted cash: Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Including Disposal Group and Discontinued Operations [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Restricted cash Restricted Cash Cash and cash equivalents and restricted cash Supplemental schedule of non-cash investing and financing activities: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Increase (decrease) in accrued capital improvements, leasing and other investing activity costs Increase (Decrease) in Accrued Capital Improvements Leasing and Other Investing Activity Costs The net change during the reporting period in future cash outflow to pay for purchases of fixed assets, construction in progress expenditures, deferred leasing costs and other investing activity costs that have occurred. Increase in fair value of derivatives applied to accumulated other comprehensive income and noncontrolling interests Increase (Decrease) in Fair Value of Derivatives Applied to Aocl and Noncontrolling Interests Increase in fair value of derivatives applied to Accumulated Other Comprehensive (Loss) Profit and noncontrolling interests Dividends/distributions payable Dividends Payable Decrease in noncontrolling interests and increase in shareholders’ equity in connection with the conversion of common units into common shares Equity Change Stock Issued During Period Value Conversion of Units Change in equity from stock issued during the period upon the conversion of units. An example of a convertible unit is an umbrella partnership real investment trust unit (UPREIT unit). Adjustments to noncontrolling interests resulting from changes in COPLP ownership Equity Change Adjustments to Additional Paid in Capital Reallocation of Minority Interest Adjustments to additional paid in capital as a result of a reallocation of a subsidiary's stockholders' equity to noncontrolling interest due to the subsidiary issuing stock. This reallocation is from a capital transaction. Increase in redeemable noncontrolling interests and decrease in equity to carry redeemable noncontrolling interests at fair value Debt, Net Debt Disclosure [Text Block] Statement of Financial Position [Abstract] Assets Assets [Abstract] Properties, net: Real Estate Investment Property, Net [Abstract] Operating properties, net Rental Properties Total properties, net Real Estate Investment Property, Net Assets held for sale, net Cash and cash equivalents Accounts receivable (net of allowance for doubtful accounts of $742 and $607, respectively) Accounts Receivable, Net Deferred rent receivable (net of allowance of $375 and $364, respectively) Deferred Rent Receivables, Net Intangible assets on real estate acquisitions, net Intangible Assets, Net (Excluding Goodwill) Deferred leasing costs (net of accumulated amortization of $29,435 and $29,560, respectively) Deferred Costs, Leasing, Net Prepaid expenses and other assets, net Prepaid Expense and Other Assets Total assets Assets Liabilities and equity Liabilities and Equity [Abstract] Liabilities: Liabilities [Abstract] Debt, net Accounts payable and accrued expenses Accounts Payable and Accrued Liabilities Rents received in advance and security deposits Advance Rent and Security Deposit Liability Amount at the balance sheet date that has been received by the entity that represents collections of cash or other assets related to real estate operations revenue for which revenue has not yet been recognized and money paid in advance to protect the provider of a product or service, such as a lessor, against damage or nonpayment by the buyer or tenant (lessee) during the term of the agreement. Such damages may include physical damage to the property, theft of property, and other contractual breaches. Security deposits held may be interest or noninterest bearing. Dividends and distributions payable Deferred revenue associated with operating leases Deferred Revenue, Leases, Net Deferred property sale Disposal Group, Including Discontinued Operation, Liabilities Capital lease obligation Capital Lease Obligations Other liabilities Other Liabilities Total liabilities Liabilities Commitments and contingencies (Note 16) Commitments and Contingencies Redeemable noncontrolling interests Redeemable Noncontrolling Interest, Equity, Carrying Amount Equity: Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Corporate Office Properties Trust’s shareholders’ equity: Stockholders' Equity Attributable to Parent [Abstract] Preferred units held by limited partner, 352,000 preferred units outstanding at March 31, 2018 and December 31, 2017 Preferred Units, Preferred Partners' Capital Accounts Common Shares of beneficial interest Additional paid-in capital Additional Paid in Capital Accumulated other comprehensive income Accumulated Other Comprehensive Income (Loss), Net of Tax Total Corporate Office Properties Trust’s shareholders’ equity Stockholders' Equity Attributable to Parent Noncontrolling interests in subsidiaries: Stockholders' Equity Attributable to Noncontrolling Interest [Abstract] Common units in COPLP Noncontrolling Interest in Operating Partnerships Preferred units in COPLP Noncontrolling Interest in Preferred Unit Holders Other consolidated entities Other Noncontrolling Interests Noncontrolling interests in subsidiaries Total equity Total liabilities, redeemable noncontrolling interests and equity Liabilities and Equity Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Balance (in units/ shares) Conversion of common units to common shares (in units/shares) Stock Issued During Period, Shares, Conversion of Units Properties, Net Real Estate Disclosure [Text Block] Income Statement [Abstract] Revenues Revenues [Abstract] Rental revenue Operating Leases, Income Statement, Lease Revenue Tenant recoveries and other real estate operations revenue Other Real Estate Revenue Expenses Operating Costs and Expenses [Abstract] Property operating expenses Direct Costs of Leased and Rented Property or Equipment Depreciation and amortization associated with real estate operations Depreciation, Depletion and Amortization, Nonproduction Construction contract and other service expenses Cost of Services General, administrative and leasing expenses Business development expenses and land carry costs Total operating expenses Costs and Expenses Operating income Operating Income (Loss) Income before equity in income of unconsolidated entities and income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income before gain on sales of real estate Gain on sales of real estate Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes Net income Net income attributable to noncontrolling interests: Net Income (Loss) Attributable to Noncontrolling Interest [Abstract] Common units in COPLP Preferred units in COPLP Noncontrolling Interest in Net Income (Loss) Preferred Unit Holders, Nonredeemable Other consolidated entities Minority Interest in Net Income (Loss) Other Including Joint Ventures Minority Interests Amount of net income (loss) for the period allocated to noncontrolling shareholders, partners, or other equity holders in one or more of the entities consolidated into the reporting entity's financial statements other than operating partnerships or interests held by preferred unit holders. Net income attributable to noncontrolling interests in consolidated entities Net Income (Loss) Attributable to Noncontrolling Interest Net income attributable to COPT Net income attributable to COPT common shareholders Earnings per common share: Dividends declared per common share/unit (in dollars per share) Common Stock, Dividends, Per Share, Declared Statement of Comprehensive Income [Abstract] Other comprehensive income Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized gain on interest rate derivatives Loss on interest rate derivatives recognized in interest expense Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax Other comprehensive income Other Comprehensive Income (Loss), Net of Tax Comprehensive income Comprehensive income attributable to noncontrolling interests Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive income attributable to COPT Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Contract with Customer, Basis of Pricing [Axis] Contract with Customer, Basis of Pricing [Axis] Contract with Customer, Basis of Pricing [Domain] Contract with Customer, Basis of Pricing [Domain] GMP Guaranteed Maximum Price [Member] Guaranteed Maximum Price [Member] FFP Fixed-price Contract [Member] Cost-plus fee Cost-plus Fee Contract [Member] Cost-plus Fee Contract [Member] Other Other Services Revenue [Member] Other Services Revenue [Member] Products and Services [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] Construction Construction Service Revenue [Member] Represents the construction service revenue from various groups. Design Design Service Revenue [Member] Design Service Revenue [Member] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Earnings Per Share [Table] Earnings Per Share [Table] Earnings Per Share [Table] Earnings Per Share [Line Items] Earnings Per Share [Line Items] [Line Items] for Earnings Per Share [Table] Summary of calculation of numerator and denominator in basic and diluted earnings per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of securities excluded from computation of diluted earnings per share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Basis of Presentation Basis of Presentation [Policy Text Block] Disclosure of accounting policy for the basis of presentation of financial statements. Reclassification Reclassification, Policy [Policy Text Block] Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Prepaid Expense and Other Assets [Abstract] Prepaid Expenses and Other Assets, Net Prepaid Expenses and Other Assets [Text Block] This block of text may be used to disclose part or all of the information related to prepaid expenses and other assets. Schedule of Real Estate Properties [Table] Schedule of Real Estate Properties [Table] Newly constructed properties placed in service Newly Constructed Properties Placed in Service [Member] Represents properties with newly constructed square feet placed in service during the reporting period. Newly redeveloped properties placed in service Newly Redeveloped Properties Placed In Service [Member] Newly Redeveloped Properties Placed In Service [Member] Properties under construction or contractually committed for construction Properties under Construction or Contractually Commited for Construction [Member] Represents real estate office properties under construction or contractually committed for construction. Properties under or approved for redevelopment Properties Under or Approved for Redevelopment [Member] Represents real estate office properties under, or approved for, redevelopment. Construction, Redevelopment and Dispositions Square footage of real estate properties (in square feet) Schedule of prepaid expenses and other assets Schedule of Prepaid and Other Assets [Table Text Block] Tabular disclosure of the carrying amounts of prepaid expenses and other assets, by major class. 2018 Operating Leases, Future Minimum Payments, Remainder of Fiscal Year 2019 Operating Leases, Future Minimum Payments, Due in Two Years 2020 Operating Leases, Future Minimum Payments, Due in Three Years 2021 Operating Leases, Future Minimum Payments, Due in Four Years 2022 Operating Leases, Future Minimum Payments, Due in Five Years Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities Excluded from Computation of Earnings Per Share, by Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Conversion of common units Exchangeable Common Units [Member] Incremental weighted average common shares attributable to common units in the operating partnership that may be exchanged for common shares. Conversion of Series I preferred units Weighted Average Convertible Preferred Unit [Member] Incremental weighted average common shares attributable to preferred units in the operating partnership that may be exchanged for common shares. Weighted average restricted stock and deferred shares Restricted Stock and Deferred Shares [Member] Restricted Stock and Deferred Shares [Member] Weighted average options Employee Stock Option [Member] Conversion of Series I preferred units Series I Preferred Units [Member] Series I preferred units representing ownership interest in the operating partnership. Antidilutive securities Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive securities excluded from computation of diluted earnings per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Lease incentives, net Incentive to Lessee Prepaid expenses Prepaid Expense Interest rate derivatives Furniture, fixtures and equipment, net Property, Plant and Equipment, Net Construction contract costs incurred in excess of billings Costs in Excess of Billings Restricted cash Non-real estate equity method investments Deferred tax asset, net Deferred Tax Assets, Net Other assets Other Assets Prepaid expenses and other assets, net Schedule of Mortgage and Other Investing Receivables [Table] Schedule of Mortgage and Other Investing Receivables [Table] Tabular disclosure of the carrying amounts of mortgage and other investing receivables, by major class. Taxable REIT Subsidiary Taxable REIT Subsidiary [Member] Represents the taxable subsidiary of the entity which is a real estate investment trust. Mortgage and Other Investing Receivables [Line Items] Mortgage and Other Investing Receivables [Line Items] Deferred tax assets, valuation allowance Deferred Tax Assets, Valuation Allowance Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Schedule of Accounts Receivable Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Contract with Customer, Asset and Liability Contract with Customer, Asset and Liability [Table Text Block] Reconciliation of Assets from Segment to Consolidated [Table] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Segment assets Operating Segments [Member] Non-operating property assets Corporate, Non-Segment [Member] Other assets Segment Reconciling Items [Member] Reconciliation of segment assets to total assets Segment Reporting, Asset Reconciling Item [Line Items] Assets Schedule of activity for redeemable noncontrolling interest Redeemable Noncontrolling Interest [Table Text Block] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Class of Stock [Line Items] Class of Stock [Line Items] Number of shares issued Sale of Stock, Number of Shares Issued in Transaction Common shares issued under forward equity sale agreements Remaining capacity under forward equity sale agreements Common Stock, Capital Shares Reserved for Future Issuance At-market stock, offering program established, remaining capacity At Market Stock, Offering Program Established, Remaining Capacity At Market Stock, Offering Program Established, Remaining Capacity 2018 Capital Leases, Future Minimum Payments, Remainder of Fiscal Year 2020 Capital Leases, Future Minimum Payments Due in Three Years 2022 Capital Leases, Future Minimum Payments Due in Five Years Total minimum rental payments Capital Leases, Future Minimum Payments Due Less: Amount representing interest Capital Leases, Future Minimum Payments, Interest Included in Payments Capital lease obligation Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments Schedule of operating properties, net Schedule of Operating Properties [Table Text Block] Tabular disclosure of net operating properties that includes carrying amount of properties and accumulated depreciation. Schedule of projects in development or held for future development Schedule of Projects in Development or held-for-future Development [Table Text Block] Tabular disclosure of properties in development or held for future development, which includes amount of land and costs incurred for construction and development. Components of assets held for sale Disclosure of Long Lived Assets Held-for-sale [Table Text Block] Schedule of information related to investments in consolidated real estate joint ventures Consolidated Real Estate, Joint Ventures [Table Text Block] Tabular disclosure of information pertaining to investments in consolidated real estate joint ventures. Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Regional Office Regional Office [Member] Regional Office [Member] Other Segments Other Segments [Member] Fort Meade/BW Corridor Fort Meade and Baltimore Washington Corridor [Member] Fort Meade and Baltimore Washington Corridor [Member] Northern Virginia Defense/IT Northern Virginia [Member] Represents Northern Virginia. Lackland Air Force Base Lackland Air Force Base [Member] Lackland Air Force Base [Member] Navy Support Locations Navy Support Locations [Member] Navy Support Locations [Member] Redstone Arsenal Redstone Arsenal [Member] Redstone Arsenal [Member] Segment financial information for real estate operations Segment Reporting Information [Line Items] Revenues from real estate operations Property operating expenses NOI from real estate operations Additions to long-lived assets Property, Plant and Equipment, Additions Transfers from non-operating properties Segment Reporting Transfers from Non Operating Properties Represents amounts transferred from non operating properties to reported segments during the period resulting from properties being placed into service. Segment assets Fair Value Measurements Fair Value Disclosures [Text Block] Document and Entity Information [Abstract] -- None. No documentation exists for this element. -- Entities [Table] Entities [Table] Entity Information [Line Items] Entity Information [Line Items] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Document Type Document Type Document Period End Date Document Period End Date Amendment Flag Amendment Flag Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Loss Contingencies [Table] Loss Contingencies [Table] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Specialty Tax Guarantee Payment Guarantee [Member] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Anne Arundel County, Maryland Anne Arundel County, Maryland [Member] Anne Arundel County, Maryland [Member] Tax Incremental Financing Bond Tax Incremental Financing Bond [Member] Tax Incremental Financing Bond [Member] Category of Contractual Obligation [Axis] Category of Item Purchased [Axis] Long-term Purchase Commitment, Category of Item Purchased [Domain] Long-term Purchase Commitment, Category of Item Purchased [Domain] New Development and Redevelopment Obligations New Development and Redevelopment Obligations [Member] New Development and Redevelopment Obligations [Member] Capital Expenditures For Operating Properties Capital Expenditures For Operating Properties [Member] Capital Expenditures For Operating Properties [Member] Third Party Construction and Development Third Party Construction and Development [Member] Third Party Construction and Development [Member] Other Obligations Other Obligations [Member] Other Obligations [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Loan amount Tax incremental financing obligation Tax Incremental Financing Obligation [Abstract] Liability recognized with regard to tax incremental financing obligation at end of current period Guarantor Obligations, Current Carrying Value Capital Leases Leases, Capital [Abstract] Capital lease term Lessee Leasing Arrangements, Capital Leases, Term of Contract Lessee Leasing Arrangements, Capital Leases, Term of Contract Purchase option Capital Leases, Purchase Option Amount Capital Leases, Purchase Option Amount Purchase obligations Contractual Obligation Environmental Indemnity Agreement Environmental Indemnity Agreement [Abstract] Number of properties which were provided environmental indemnifications Property Subject to Environmental Indemnity, Number of Properties Represents the number of properties in New Jersey which were provided environmental indemnifications by the entity. Maximum additional costs agreed to be paid by the entity under environmental indemnification agreement Maximum Additional Costs Agreed To Be Paid By The Entity Under Environmental Indemnification Agreement Maximum additional costs agreed to be paid by the entity under environmental indemnification agreement Allowance for doubtful accounts - AR Allowance for Doubtful Accounts Receivable Deferred rent receivable Allowance for Doubtful Accounts, Deferred Rent Valuation allowance for deferred rent receivables attributable to, for instance, credit risk associated with a lessee. Deferred leasing costs, accumulated amortization Deferred Costs, Leasing, Accumulated Amortization Common Shares of beneficial interest, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common Shares of beneficial interest, shares authorized Common Stock, Shares Authorized Common Shares of beneficial interest, shares issued Common Stock, Shares, Issued Common Shares of beneficial interest, shares outstanding Preferred Units, Outstanding Future minimum rental payments for operating leases Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] Future minimum lease payments for capital leases Schedule of Future Minimum Lease Payments for Capital Leases [Table Text Block] Organization Nature of Operations [Text Block] Schedule of key terms and fair values of interest rate swap derivatives Schedule of Derivative Instruments [Table Text Block] Schedule of fair value and balance sheet classification of interest rate derivatives Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Schedule of effect of interest rate derivatives on consolidated statements of operations and comprehensive income Derivative Instruments, Gain (Loss) [Table Text Block] Schedule of fair value assets and liabilities measured on recurring basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Construction Contract Revenue Construction Contract Revenue [Member] Construction Contract Revenue [Member] Number of compensation arrangement types Revenue from Contract with Customer, Construction Contracts, Number of Compensation Arrangements Types Revenue from Contract with Customer, Construction Contracts, Number of Compensation Arrangement Types Performance obligations satisfied or already satisfied Contract with Customer, Performance Obligation Satisfied in Previous Period Customers payment period Revenue from Contract with Customer, Payment Period Revenue from Contract with Customer, Payment Period Remaining performance obligations Revenue, Remaining Performance Obligation Change in Accounts Receivable Change in Accounts Receivable [Abstract] Change in Accounts Receivable Beginning balance Ending balance Change in Contract with Customer, Asset Change in Contract with Customer, Asset [Abstract] Beginning balance Contract with Customer, Asset, Net Ending balance Change in Contract with Customer, Liability Change in Contract with Customer, Liability [Abstract] Beginning balance Contract with Customer, Liability Ending balance Revenue recognized included in beginning balance Contract with Customer, Liability, Revenue Recognized Land Land [Member] Buildings and improvements Building and Building Improvements [Member] Gross SEC Schedule III, Real Estate, Gross Less: accumulated depreciation Real Estate Investment Property, Accumulated Depreciation Operating properties, net Schedule of segment financial information for real estate operations Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of reconciliation of segment revenues to total revenues Reconciliation of Revenue from Segments to Consolidated [Table Text Block] Schedule of reconciliation of unconsolidated joint venture net operating income to equity in income of unconsolidated entities Schedule of Reconciliation of Unconsolidated Joint Venture Net Operating Income to Equity in Income of Unconsolidated Entities [Table Text Block] Schedule of Reconciliation of Unconsolidated Joint Venture Net Operating Income to Equity in Income of Unconsolidated Entities [Table Text Block] Schedule of computation of net operating income from service operations Schedule of Net Operating Income from Segments from Service Operations [Table Text Block] Tabular disclosure of the computation of net operating income from service operations. Schedule of reconciliation of net operating income from real estate operations and service operations to (loss) income from continuing operations Reconciliation of Operating Profit (Loss) from Segments to Consolidated Income from Continuing Operations [Table Text Block] Tabular disclosure of all significant reconciling items in the reconciliation of total profit or loss from reportable segments to the entity's consolidated income from continuing operations. Schedule of reconciliation of segment assets to total assets Reconciliation of Assets from Segment to Consolidated [Table Text Block] Investing Receivables Financing Receivables [Text Block]