Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	Nov. 30, 2015	Dec. 31, 2015	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate [Line Items]
Depreciation expense (including amortization of the capital lease asset)					$ 365,300	$ 783,500	$ 523,800
Impairment Charge				$ 10,400
Period from cessation of major construction to consider construction project as complete and available for occupancy, maximum (in years)					1 year
Weighted average amortization period for above-market leases (in years)					6 years
Weighted average amortization period for below-market leases (in years)					23 years 4 months
Weighted average amortization for in-place lease costs (in years)					16 years 4 months
Amortization of above and below Market Leases					$ 20,300	196,200	38,700
Reduction in interest expense from amortization of above-market rate mortgages					800	2,800	$ 2,300
Identified intangible assets (included in other assets):
Gross amount					325,880	651,099
Accumulated amortization					(277,038)	(410,930)
Net					48,842	240,169
Identified intangible liabilities (included in deferred revenue):
Gross amount					540,283	655,930
Accumulated amortization					(402,583)	(464,749)
Net					137,700	$ 191,181
Net intangible assets reclassed to assets held for sale					13,900
Net intangible liabilities reclassed to liabilities relates to assets held for sale					4,100
Other Identifiable Assets
Estimated annual amortization
2018					9,700
2019					7,716
2020					6,783
2021					3,927
2022					2,365
Acquired above-market leases, net of acquired below-market leases
Below Market Lease, Net, Amortization Income, Fiscal Year Maturity [Abstract]
2018					(13,853)
2019					(13,250)
2020					(12,432)
2021					(3,950)
2022					$ (3,647)
600 Lexington Avenue
Real Estate [Line Items]
Purchase price fair value adjustment realized	$ 40,078	$ 40,100
Maximum | Above-market leases
Real Estate [Line Items]
Estimated useful life of other intangible assets (in years)					14 years
Maximum | Below-market leases
Real Estate [Line Items]
Estimated useful life of other intangible assets (in years)					14 years
Maximum | In-place leases
Real Estate [Line Items]
Estimated useful life of other intangible assets (in years)					14 years
Minimum | Above-market leases
Real Estate [Line Items]
Estimated useful life of other intangible assets (in years)					1 year
Minimum | Below-market leases
Real Estate [Line Items]
Estimated useful life of other intangible assets (in years)					1 year
Minimum | In-place leases
Real Estate [Line Items]
Estimated useful life of other intangible assets (in years)					1 year
Building (fee ownership)
Real Estate [Line Items]
Estimated useful life (in years)					40 years
Building (leasehold interest) | Maximum
Real Estate [Line Items]
Estimated useful life (in years)					40 years
Furniture and fixtures | Maximum
Real Estate [Line Items]
Estimated useful life (in years)					7 years
Furniture and fixtures | Minimum
Real Estate [Line Items]
Estimated useful life (in years)					4 years
Building | Maximum
Real Estate [Line Items]
Estimated useful life (in years)					40 years
Building | Minimum
Real Estate [Line Items]
Estimated useful life (in years)					3 years
140-150 Grand Street
Real Estate [Line Items]
Impairment Charge			$ 19,200
388-390 Greenwich Street
Real Estate [Line Items]
Amortization of below market leases					$ 172,400