Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2015
Accounting Policies [Abstract]
Basis of quarterly presentation and principles of combination	Basis of Quarterly Presentation and Principles of Consolidation The accompanying unaudited condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”), for interim financial information, and with the rules and regulations of the Securities and Exchange Commission (the "SEC"). Accordingly, certain information and footnote disclosures required by GAAP for complete financial statements have been condensed or omitted in accordance with such rules and regulations. In the opinion of management, all adjustments and eliminations (including intercompany balances and transactions), consisting of normal recurring adjustments, considered necessary for the fair presentation of the financial statements have been included. For purposes of comparison, certain items shown in the 2014 unaudited condensed consolidated financial statements have been reclassified to conform to the presentation used for 2015.
Consolidation	The results of operations for the periods presented are not necessarily indicative of the results that may be expected for the corresponding full years. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the financial statements for the year ended December 31, 2014 contained in our Annual Report on Form 10-K. We do not consider our business to be subject to material seasonal fluctuations, except that our observatory business is subject to tourism seasonality.
Consolidations for variable interest entity	The primary beneficiary of a VIE 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. The primary beneficiary is required to consolidate the VIE. We had no VIEs as of September 30, 2015 and December 31, 2014. We will assess the accounting treatment for each investment we may have in the future. This assessment will include a review of each entity’s organizational agreement to determine which party has what rights and whether those rights are protective or participating. For all VIEs, we will review such agreements in order to determine which party has the power to direct the activities that most significantly impact the entity’s economic performance and benefit. In situations where we or our partner could approve, among other things, the annual budget, or leases that cover more than a nominal amount of space relative to the total rentable space at each property, we would not consolidate the investment as we consider these to be substantive participation rights that result in shared power of the activities that would most significantly impact the performance and benefit of such joint venture investment. A non-controlling interest in a consolidated subsidiary is defined as the portion of the equity (net assets) in a subsidiary not attributable, directly or indirectly, to a parent. Non-controlling interests are required to be presented as a separate component of equity in the condensed consolidated balance sheets and in the condensed consolidated statements of income by requiring earnings and other comprehensive income to be attributed to controlling and non-controlling interests.
Accounting estimates	Accounting Estimates The preparation of the condensed consolidated financial statements in accordance with GAAP requires management to use estimates and assumptions that in certain circumstances affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities, and the reported revenues and expenses. Significant items subject to such estimates and assumptions include allocation of the purchase price of acquired real estate properties among tangible and intangible assets, determination of the useful life of real estate properties and other long-lived assets, valuation and impairment analysis of commercial real estate properties and other long-lived assets, estimate of percentage of completion on construction contracts, valuation of the allowance for doubtful accounts, and valuation of exchangeable senior unsecured notes and equity based compensation. These estimates are prepared using management’s best judgment, after considering past, current, and expected events and economic conditions. Actual results could differ from those estimates.
Derivative Instruments	Derivative Instruments We are exposed to the effect of interest rate changes and manage these risks by following policies and procedures including the use of derivatives. To manage exposure to interest rates, derivatives are used primarily to fix the rate on debt based on floating-rate indices. We record all derivatives on the balance sheet at fair value. We incorporate credit valuation adjustments to appropriately reflect both our own nonperformance risk and the respective counterparty’s nonperformance risk in the fair value measurements. For derivatives that qualify as cash flow hedges, we report the effective portion of changes in the fair value of a derivative designated as a hedge as part of other comprehensive income (loss) and subsequently reclassify the effective portion into income in the period that the hedged item affects income. We account for the ineffective portion of changes in the fair value of a derivative directly in income. Reported net income and equity may increase or decrease prospectively, depending on future levels of interest rates and other variables affecting the fair values of derivative instruments and hedged items, but will have no effect on cash flows. We measure the credit risk of our derivative instruments that are subject to master netting agreements on a net basis by counterparty portfolio.
Recently issued or adopted accounting standards	Recently Issued or Adopted Accounting Standards During September 2015, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2015-16, Business Combinations (Topic 805): Simplifying the Accounting for Measurement-Period Adjustments, which requires an acquirer to recognize adjustments to provisional amounts that are identified during the measurement period in the reporting period in which the adjustment amounts are determined. Previously, adjustments to provisional amounts were applied retrospectively. This update requires an entity to present separately on the face of the income statement or disclose in the notes the portion of the amount recorded in current-period earnings by line item that would have been recorded in previous reporting periods if the adjustment to the provisional amounts had been recognized as of the acquisition date. ASU No. 2015-16 will be effective for fiscal years beginning January 1, 2016 and subsequent interim periods. The implementation of this update is not expected to cause any material changes to our consolidated financial statements. During June 2015, the FASB issued ASU No. 2015-10, Technical Corrections and Improvements, which contains amendments that represent changes to clarify the FASB Accounting Standards Codification (“Codification”), correct unintended application of guidance, or make minor improvements to the Codification that are not expected to have a significant effect on current accounting practice or create a significant administrative cost to most entities. The amendments in ASU No. 2015-10 that require transition guidance are effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2015. All other amendments will be effective upon its issuance. The implementation of this update did not and is not expected to cause any material changes to our consolidated financial statements. During April 2015, the FASB issued ASU No. 2015-03, Interest - Imputation of Interest (Subtopic 835-30): Simplifying the Presentation of Debt Issuance Costs, which amends the requirements for the presentation of debt issuance costs and requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. ASU No. 2015-03 is effective for fiscal years, beginning after December 15, 2015 and interim periods within those fiscal years. The implementation of this update is not expected to cause any material changes to our consolidated financial statements other than the reclassification of debt issuance costs from assets to a reduction of liabilities on our consolidated balance sheets. ASU No. 2015-03 was amended in August 2015 by ASU No. 2015-15, Interest - Imputation of Interest (Subtopic 835-30) - Presentation and Subsequent Measurement of Debt Issuance Costs Associated with Line-of-Credit Arrangements, to add to the Codification SEC staff guidance that the SEC staff will not object to an entity presenting the costs of securing line-of-credit arrangements as an asset, regardless of whether there are any outstanding borrowings. The SEC Observer to the Emerging Issues Task Force announced the staff guidance in response to questions that arose after the FASB issued ASU No. 2015-03. During February 2015, the FASB issued ASU No. 2015-02, Consolidation (Topic 810) - Amendments to the Consolidation Analysis, which amends the criteria for determining which entities are considered VIEs, amends the criteria for determining if a service provider possesses a variable interest in a VIE and ends the deferral granted to investment companies for application of the VIE consolidation model. ASU No. 2015-02 is effective for fiscal years, beginning after December 15, 2015 and interim periods within those fiscal years. We are evaluating the impact of adopting this new accounting standard on our financial statements. During January 2015, the FASB issued ASU No. 2015-01, Income Statement - Extraordinary and Unusual Items, which eliminates the concept of extraordinary items. However, the presentation and disclosure requirements for items that are either unusual or in nature or infrequent in occurrence remain and will be expanded to include items that are both unusual in nature and infrequent in occurrence. ASU No. 2015-01 is effective for periods beginning after December 15, 2015. We are evaluating the impact of adopting this new accounting standard on our financial statements. During May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers, which will replace all current GAAP guidance related to revenue recognition and eliminate all industry-specific guidance. The new revenue recognition standard provides a unified model to determine when and how revenue is recognized. The core principle is that a company should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. This guidance will be effective beginning in 2017 and can be applied either retrospectively to each period presented or as a cumulative-effect adjustment as of the date of adoption. We are evaluating the impact of adopting this new accounting standard on our financial statements. ASU No. 2014-09 was amended in August 2015 by ASU No. 2015-14 Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which defers the effective date of ASU No. 2014-09 for all entities by one year. Public business entities, certain not-for-profit entities, and certain employee benefit plans should apply the guidance in ASU No. 2014-09 to annual reporting periods beginning after December 15, 2017, including interim reporting periods within that reporting period. Earlier application is permitted only as of annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. During April 2014, the FASB issued ASU No. 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity, which changes the definition of a discontinued operation to include only those disposals of components of an entity that represent a strategic shift that has (or will have) a major effect on an entity’s operations and financial results. This guidance is effective prospectively for fiscal years beginning after December 15, 2014. The adoption of ASU No. 2014-08 did not have a material impact on our financial statements.

Disclosure of accounting policy for consolidation to describe the significant judgments and assumptions made in determining whether a variable interest held by the entity requires the variable interest entity to be consolidated and (or) disclose information about its involvement with the variable interest entity; the methodology used by the entity for determining whether or not it is the primary beneficiary of the variable interest entity; and the significant factors considered and judgments made in determining that the power to direct the activities that significantly impact the economic performance of the variable interest entity are shared (as defined).