Significant Accounting Policies (Policies)	3 Months Ended
Significant Accounting Policies (Policies)	Mar. 31, 2018
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include our accounts and those of our subsidiaries, which are wholly-owned or controlled by us. Entities which we do not control through our voting interest and entities which are variable interest entities, but where we are not the primary beneficiary, are accounted for under the equity method. See Note 5, "Debt and Preferred Equity Investments" and Note 6, "Investments in Unconsolidated Joint Ventures." All significant intercompany balances and transactions have been eliminated. We consolidate a variable interest entity, or VIE, in which we are considered the primary beneficiary. The primary beneficiary is the entity that has (i) the power to direct the activities that most significantly impact the entity's economic performance and (ii) the obligation to absorb losses of the VIE or the right to receive benefits from the VIE that could be significant to the VIE.
Investment in Commercial Real Estate Properties	Investment in Commercial Real Estate Properties We allocate the purchase price of real estate to land and building (inclusive of tenant improvements) and, if determined to be material, intangibles, such as the value of above- and below-market leases and origination costs associated with the in-place leases. We depreciate the amount allocated to building (inclusive of tenant improvements) over their estimated useful lives, which generally range from three to 40 years. We amortize the amount allocated to the above- and below-market leases over the remaining term of the associated lease, which generally range from one to 14 years, and record it as either an increase (in the case of below-market leases) or a decrease (in the case of above-market leases) to rental income. We amortize the amount allocated to the values associated with in-place leases over the expected term of the associated lease, which generally ranges from one to 14 years. If a tenant vacates its space prior to the contractual termination of the lease and no rental payments are being made on the lease, any unamortized balance of the related intangible will be written off. The tenant improvements and origination costs are amortized as an expense over the remaining life of the lease (or charged against earnings if the lease is terminated prior to its contractual expiration date). We assess fair value of the leases based on estimated cash flow projections that utilize appropriate discount and capitalization rates and available market information. Estimates of future cash flows are based on a number of factors including the historical operating results, known trends, and market/economic conditions that may affect the property. To the extent acquired leases contain fixed rate renewal options that are below-market and determined to be material, we amortize such below-market lease value into rental income over the renewal period.
Fair Value Measurements	Fair Value Measurements We are required to disclose fair value information with regard to our financial instruments, whether or not recognized in the consolidated balance sheets, for which it is practical to estimate fair value. The FASB guidance defines fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants on the measurement date. We measure and/or disclose the estimated fair value of financial assets and liabilities based on a hierarchy that distinguishes between market participant assumptions based on market data obtained from sources independent of the reporting entity and the reporting entity’s own assumptions about market participant assumptions. This hierarchy consists of three broad levels: Level 1 - quoted prices (unadjusted) in active markets for identical assets or liabilities that the reporting entity can access at the measurement date; Level 2 - inputs other than quoted prices included within Level 1, that are observable for the asset or liability, either directly or indirectly; and Level 3 - unobservable inputs for the asset or liability that are used when little or no market data is available. We follow this hierarchy for our assets and liabilities measured at fair value on a recurring and nonrecurring basis. In instances in which the determination of the fair value measurement is based on inputs from different levels of the fair value hierarchy, the level in the fair value hierarchy within which the entire fair value measurement falls is based on the lowest level of input that is significant to the fair value measurement in its entirety. Our assessment of the significance of the particular input to the fair value measurement in its entirety requires judgment and considers factors specific to the asset or liability. We determine other than temporary impairment in real estate investments and debt and preferred equity investments, including intangibles primarily utilizing cash flow projections that apply, among other things, estimated revenue and expense growth rates, discount rates and capitalization rates, as well as sales comparison approach, which utilizes comparable sales, listings and sales contracts. All of which are classified as Level 3 inputs. As of March 31, 2018, we held notes receivable with an outstanding principal balance of $250.5 million and an accrued interest balance of $7.7 million, which were purchased at par, and were in maturity default at the time of acquisition. In August 2017, the Company determined that it was probable that the loans would not be repaid in full and therefore, the loans were put on non-accrual status. The Company has initiated proceedings to foreclose on the property, and expects to take control of the property unless the buyer is able to repay the principal and interest, including default interest and fees, on the notes receivable in full prior to the completion of the foreclosure process. We believe the collateral value is sufficient to recover the carrying amounts of the notes receivable. In January 2018, the partnership agreement for our investment in 919 Third Avenue was modified resulting in the Company no longer having a controlling interest in this investment. As a result the investment was deconsolidated as of January 1, 2018. The Company recorded its non-controlling interest at fair value resulting in a $49.3 million fair value adjustment in the consolidated statement of operations. This fair value was determined using a third party valuation which primarily utilized cash flow projections that apply, among other things, estimated revenue and expense growth rates, discount rates and capitalization rates, as well as sales comparison approach, which utilizes comparable sales, listings and sales contracts. All of which are classified as Level 3 inputs. Marketable securities classified as Level 1 are derived from quoted prices in active markets. The valuation technique used to measure the fair value of marketable securities classified as Level 2 were valued based on quoted market prices or model driven valuations using the significant inputs derived from or corroborated by observable market data. Marketable securities in an unrealized loss position are not considered to be other than temporarily impaired. We do not intend to sell these securities and it is not more likely than not that we will be required to sell the investments before recovery of their amortized cost bases. The fair value of derivative instruments is based on current market data received from financial sources that trade such instruments and are based on prevailing market data and derived from third party proprietary models based on well-recognized financial principles and reasonable estimates about relevant future market conditions, which are classified as Level 2 inputs. The financial assets and liabilities that are not measured at fair value on our consolidated balance sheets include cash and cash equivalents, restricted cash, accounts receivable, accounts payable and accrued expenses, debt and preferred equity investments, mortgages and other loans payable and other secured and unsecured debt. The carrying amount of cash and cash equivalents, restricted cash, accounts receivable, and accounts payable and accrued expenses reported in our consolidated balance sheets approximates fair value due to the short term nature of these instruments. The fair value of debt and preferred equity investments, which is classified as Level 3, is estimated by discounting the future cash flows using current interest rates at which similar loans with the same maturities would be made to borrowers with similar credit ratings. The fair value of borrowings, which is classified as Level 3, is estimated by discounting the contractual cash flows of each debt instrument to their present value using adjusted market interest rates, which is provided by a third-party specialist.
Investment in Marketable Securities	Investment in Marketable Securities At acquisition, we designate a debt security as held-to-maturity, available-for-sale, or trading. As of March 31, 2018, we did not have any debt securities designated as held-to-maturity or trading. We account for our available-for-sale debt securities at fair value pursuant to Accounting Standards Codification, or ASC, 820-10, with the net unrealized gains or losses reported as a component of accumulated other comprehensive income or loss. Any unrealized losses that are determined to be other-than-temporary are recognized in earnings up to their credit component. The Company adopted ASU 2016-01 effective January 1, 2018 which required entities to measure investments in equity securities at fair value and recognize any changes in fair value in net income. Upon adoption we did not hold investments in equity securities and therefore did not record a cumulative-effect adjustment. We did not hold investments in equity securities as of March 31, 2018. The cost of bonds and marketable securities sold is determined using the specific identification method.
Investments in Unconsolidated Joint Ventures	Investments in Unconsolidated Joint Ventures We assess our investments in unconsolidated joint ventures for recoverability and if it is determined that a loss in value of the investment is other than temporary, we write down the investment to its fair value. We evaluate our equity investments for impairment based on the joint ventures' projected discounted cash flows.
Reserve for Possible Credit Losses	Reserve for Possible Credit Losses The reserve for possible credit losses in connection with debt and preferred equity investments is the charge to earnings to increase the allowance for possible credit losses to the level that we estimate to be adequate, based on Level 3 data, considering delinquencies, loss experience and collateral quality. Other factors considered include geographic trends, product diversification, the size of the portfolio and current economic conditions. Based upon these factors, we establish a provision for possible credit loss on each individual investment. When it is probable that we will be unable to collect all amounts contractually due, the investment is considered impaired. Where impairment is indicated on an investment that is held to maturity, a valuation allowance is measured based upon the excess of the recorded investment amount over the fair value of the collateral. Any deficiency between the carrying amount of an asset and the calculated value of the collateral is charged to expense. We continue to assess or adjust our estimates based on circumstances of a loan and the underlying collateral. If additional information reflects increased recovery of our investment, we will adjust our reserves accordingly. There were no loan reserves recorded during the three months ended March 31, 2018 and 2017. Debt and preferred equity investments held for sale are carried at the lower of cost or fair market value using available market information obtained through consultation with dealers or other originators of such investments as well as discounted cash flow models based on Level 3 data pursuant to ASC 820-10. As circumstances change, management may conclude not to sell an investment designated as held for sale. In such situations, the investment will be reclassified at its net carrying value to debt and preferred equity investments held to maturity. For these reclassified investments, the difference between the current carrying value and the expected cash to be collected at maturity will be accreted into income over the remaining term of the investment.
Income Taxes	Income Taxes SL Green is taxed as a REIT under Section 856(c) of the Code. As a REIT, SL Green generally is not subject to Federal income tax. To maintain its qualification as a REIT, SL Green must distribute at least 90% of its REIT taxable income to its stockholders and meet certain other requirements. If SL Green fails to qualify as a REIT in any taxable year, SL Green will be subject to Federal income tax on its taxable income at regular corporate rates. SL Green may also be subject to certain state, local and franchise taxes. Under certain circumstances, Federal income and excise taxes may be due on its undistributed taxable income. The Operating Partnership is a partnership and, as a result, all income and losses of the partnership are allocated to the partners for inclusion in their respective income tax returns. The only provision for income taxes included in the consolidated statements of operations relates to the Operating Partnership’s consolidated taxable REIT subsidiaries. The Operating Partnership may also be subject to certain state, local and franchise taxes. Pursuant to amendments to the Code that became effective January 1, 2001, we have elected, and may elect in the future, to treat certain of our existing or newly created corporate subsidiaries as taxable REIT subsidiaries, or TRSs. In general, TRSs may perform non-customary services for the tenants of the Company, hold assets that we cannot hold directly and generally may engage in any real estate or non-real estate related business. The TRSs generate income, resulting in Federal and state income tax liability for these entities. During the three months ended March 31, 2018, we recorded Federal, state and local tax provisions of $0.5 million. We follow a two-step approach for evaluating uncertain tax positions. Recognition (step one) occurs when an enterprise concludes that a tax position, based solely on its technical merits, is more-likely-than-not to be sustained upon examination. Measurement (step two) determines the amount of benefit that is more-likely-than-not to be realized upon settlement. Derecognition of a tax position that was previously recognized would occur when a company subsequently determines that a tax position no longer meets the more-likely-than-not threshold of being sustained. The use of a valuation allowance as a substitute for derecognition of tax positions is prohibited.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that potentially subject us to concentrations of credit risk consist primarily of cash investments, debt and preferred equity investments and accounts receivable. We place our cash investments with high quality financial institutions. The collateral securing our debt and preferred equity investments is located in the New York metropolitan area. See Note 5, "Debt and Preferred Equity Investments." We perform ongoing credit evaluations of our tenants and require most tenants to provide security deposits or letters of credit. Though these security deposits and letters of credit are insufficient to meet the total value of a tenant's lease obligation, they are a measure of good faith and a source of funds to offset the economic costs associated with lost revenue and the costs associated with re-tenanting a space.
Reclassification	Reclassification Certain prior year balances have been reclassified to conform to our current year presentation.
Accounting Standards Updates	Accounting Standards Updates In February 2018, the FASB issued Accounting Standard Update (ASU) No. 2018-03, Technical Corrections and Improvements to Financial Instruments- Overall (Subtopic 825-10), Recognition and Measurement of Financial Assets and Financial Liabilities. These amendments provide additional guidance related to equity securities without a readily determinable fair value, forward contracts and options purchased on those equity securities and fair value option liabilities. The guidance will be effective for the Company in the interim period beginning after June 15, 2018. The Company has not yet adopted this new guidance and does not expect it to have a material impact on the Company’s consolidated financial statements when the new standard is implemented. In August 2017, the FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815), Targeted Improvements to Accounting for Hedging Activities. The amendments in the new standard will permit more flexibility in hedging interest rate risk for both variable rate and fixed rate financial instruments. The standard will also enhance the presentation of hedge results in the financial statements. The guidance is effective for fiscal years beginning after December 15, 2018. Early adoption is permitted. The Company has not yet adopted the guidance, and does not expect a material impact on the Company’s consolidated financial statements when the new standard is implemented. In May 2017, the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718), Scope of Modification Accounting. The guidance clarifies the changes to the terms or conditions of a share-based payment award that require an entity to apply modification accounting in Topic 718. The Company adopted the guidance on January 1, 2018 and it had no impact on the Company's consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The guidance requires that the statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash. As a result, entities will no longer present transfers between these items on the statement of cash flows. The Company adopted the guidance on January 1, 2018 and has included the changes in restricted cash when reconciling the beginning-of-period and end-of-period total amounts on the statement of cash flows. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The guidance changes how entities will measure credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. The guidance replaces the current ‘incurred loss’ model with an ‘expected loss’ approach. The guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted after December 15, 2018. The Company has not yet adopted this new guidance and is currently evaluating the impact of adopting this new accounting standard on the Company’s consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, Leases. The guidance requires lessees to recognize lease assets and lease liabilities for those leases classified as operating leases under the previous standard. Depending on the lease classification, lessees will recognize expense based on the effective interest method for finance leases or on a straight-line basis for operating leases. The accounting applied by a lessor is largely unchanged from that applied under the previous standard. However, the presentation and disclosure in the financial statements of non-lease components such as charges to tenants for a building’s operating expenses has been updated. In January 2018, the FASB issued an exposure draft, Leases (Topic 842) Targeted Improvements, which would provide lessors with a practical expedient to not separate non-lease components from the related lease component under certain conditions. At the March 2018 Board meeting, the Board approved an amendment to the practical expedient. We anticipate the majority of our leases would qualify for the amended practical expedient. We expect this amended guidance to be finalized in 2018 and, as a result, we would adopt the practical expedient. Another impact is the measurement and presentation of ground leases under which the Company is lessee. The Company is required to record a liability for the obligation to make payments under the lease and an asset for the right to use the underlying asset during the lease term and will also apply the new expense recognition requirements given the lease classification. The Company is currently quantifying these impacts. The guidance is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early adoption is permitted. The Company expects to adopt this guidance January 1, 2019 and will apply the modified retrospective approach. In January 2016, the FASB issued ASU 2016-01 (Subtopic 825-10), Recognition and Measurement of Financial Assets and Financial Liabilities. The guidance requires entities to measure equity investments that do not result in consolidation and are not accounted for under the equity method at fair value through earnings, to record changes in instrument-specific credit risk for financial liabilities measured under the fair value option in other comprehensive income, use the exit price notion when measuring an instrument’s fair value for disclosure and to separately present financial assets and liabilities by measurement category and form of instrument on the balance sheet or in the notes to the financial statements. The Company adopted the guidance effective January 1, 2018, and it had no impact on the Company’s consolidated financial statements. In May 2014, the FASB issued a new comprehensive revenue recognition guidance which requires us to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which we expect to be entitled in exchange for those goods and services (ASU 2014-09). The FASB also issued implementation guidance in March 2016, April 2016 and May 2016 - ASU’s 2016-08, 2016-10 and 2016-12, respectively. The Company adopted this guidance on January 1, 2018. Since the Company’s revenue is related to leasing activities, the adoption of this guidance did not have a material impact on the consolidated financial statements. The new guidance is applicable to service contracts with joint ventures for which the Company earns property management fees, leasing commissions and development and construction fees. The adoption of this new guidance did not change the accounting for these fees as the pattern of recognition of revenue does not change with the new guidance. We will continue to recognize revenue over time on these contracts because the customer simultaneously receives and consumes the benefits provided by our performance. In February 2017, the FASB issued ASU No. 2017-05 to clarify the scope of asset derecognition guidance in Subtopic 610-20, which also provided guidance on accounting for partial sales of nonfinancial assets. Subtopic 610-20 was issued in May 2014 as part of ASU 2014-09. The Company adopted this guidance on January 1, 2018, and applied the modified retrospective approach. The Company elected to adopt the practical expedient under ASC 606, Revenue from Contracts with Customers, which allows an entity to apply the guidance only to contracts with non-customers that are open based on ASU 360-20, Real Estate Sales, (i.e. failed sales) as of the adoption date. The Company had one open contract in 2017 with a non-customer that was evaluated under ASC 610-20. The Company entered into an agreement to sell a portion of their interest in an entity that held a controlling interest in the property at 1515 Broadway. Upon execution of the agreement in 2017, the transaction was evaluated under ASC 360-20, Real Estate Sales, and did not meet the criteria for sale accounting. Upon adoption of ASC 606, this contract met the criteria for sale accounting under ASC 610-20. Through the sale, the Company no longer retains a controlling interest, as defined in ASC 810, Consolidation, and the impact of this adjustment is a gain of $0.6 billion from the sale of the partial interest and related step-up in basis to fair value of the non-controlling interest retained. This was recorded in the first quarter of 2018 as an adjustment to beginning retained earnings.