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Significant Accounting Policies - Concentrations of Credit Risk/Accounting Standards Updates (Details) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) Tenant	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($)
Concentration of Credit Risk
Increase in total assets	[1]	$ 13,455,004	$ 13,455,004		$ 13,982,904
Increase in total liabilities	[1]	$ 6,418,801	$ 6,418,801		$ 6,629,761
Annualized rent | Customer concentration
Concentration of Credit Risk
Number of tenants (tenants) | Tenant			1
Maximum percentage of annualized rent for any one tenant not individually disclosed (percent) (more than)		5.00%	5.00%
Annualized rent | 11 Madison Avenue | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)		7.20%
Annualized rent | 420 Lexington Avenue | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)		6.40%
Annualized rent | 1185 Avenue of the Americas | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)		6.20%
Annualized rent | 1515 Broadway | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)		5.80%
Annualized rent | 1 Madison Ave | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)		5.60%
Tenant 1 | Annualized rent | Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)			8.10%
1515 Broadway | Accounting Standards Update 2017-05
Concentration of Credit Risk
Gain on disposal			$ 600,000
Scenario, Forecast | Minimum | Accounting Standards Update 2016-02
Concentration of Credit Risk
Increase in total assets				$ 400,000
Increase in total liabilities				400,000
Scenario, Forecast | Maximum | Accounting Standards Update 2016-02
Concentration of Credit Risk
Increase in total assets				500,000
Increase in total 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, $34.6 million and $330.9 million of accumulated depreciation, $991.3 million and $221.0 million of other assets included in other line items, $141.0 million and $628.9 million of real estate debt, net, $0.4 million and $2.5 million of accrued interest payable, $43.4 million and $42.8 million of capital lease obligations, and $203.4 million and $56.8 million of other liabilities included in other line items as of September 30, 2018 and December 31, 2017, respectively.

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