Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Real Estate [Line Items]
Depreciation expense (including amortization of the capital lease asset)	$ 242,800,000	$ 365,300,000	$ 783,500,000
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	1 year 9 months 18 days
Weighted average amortization period for below-market leases (in years)	4 years 7 months 6 days
Weighted average amortization for in-place lease costs	5 years 9 months 18 days
Amortization of above and below Market Leases	$ 6,800,000	20,300,000	196,200,000
Reduction in interest expense from amortization of above-market rate mortgages	0	800,000	2,800,000
Identified intangible assets (included in other assets):
Gross amount	266,540,000	325,880,000
Accumulated amortization	(241,040,000)	(277,038,000)
Net	25,500,000	48,842,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	276,245,000	540,283,000
Accumulated amortization	(253,767,000)	(402,583,000)
Net	22,478,000	137,700,000
Net intangible assets reclassed to assets held for sale	0	13,900,000
Net intangible liabilities reclassed to liabilities relates to assets held for sale	0	$ 4,100,000
Other Identifiable Assets
Estimated annual amortization
2019	9,825,000
2020	4,817,000
2021	3,454,000
2022	1,892,000
2023	1,507,000
Acquired above-market leases, net of acquired below-market leases
Below Market Lease, Net, Amortization Income, Fiscal Year Maturity [Abstract]
2019	(5,227,000)
2020	(3,655,000)
2021	(1,631,000)
2022	(1,328,000)
2023	$ (749,000)
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
388-390 Greenwich Street
Real Estate [Line Items]
Amortization of below market leases			$ 172,400,000