Significant Accounting Policies - Accounting Standards Update (Details) - USD ($) $ in Thousands			3 Months Ended
		Jan. 01, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets	[1]			$ 12,751,358	$ 13,982,904
Liabilities	[1]			$ 6,115,271	$ 6,629,761
1515 Broadway | Accounting Standards Update 2017-05
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gain on disposal			$ 600,000
Scenario, Forecast | Accounting Standards Update 2018-11
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated decrease in net income		$ 10,000
Minimum | Scenario, Forecast | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets		400,000
Liabilities		400,000
Maximum | Scenario, Forecast | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets		500,000
Liabilities		$ 500,000

[1] The Company's consolidated balance sheets include assets and liabilities of consolidated variable interest entities ("VIEs"). See Note 2. The consolidated balance sheets include the following amounts related to our consolidated VIEs, excluding the Operating Partnership: $110.0 million and $398.0 million of land, $0.3 billion and $1.4 billion of building and improvements, $2.0 million and $2.0 million of building and leasehold improvements, $47.4 million and $47.4 million of properties under capital lease, $42.2 million and $330.9 million of accumulated depreciation, $721.3 million and $221.0 million of other assets included in other line items, $140.8 million and $628.9 million of real estate debt, net, $0.4 million and $2.5 million of accrued interest payable, $43.6 million and $42.8 million of capital lease obligations, and $18.4 million and $56.8 million of other liabilities included in other line items as of December 31, 2018 and December 31, 2017, respectively.