Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Real Estate [Line Items]
Weighted average amortization period, above-market lease costs	1 year 8 months 12 days
Weighted average amortization period, below-market lease costs	2 years 9 months 18 days
Weighted average amortization period, in-place lease costs	3 years 6 months
Period from cessation of major construction activity to completion of construction project, maximum	1 year
Depreciation expense (including amortization of the capital lease asset)	$ 233,500,000	$ 242,800,000	$ 365,300,000
Amortization of above and below Market Leases	4,500,000	6,800,000	20,300,000
Reduction in interest expense from amortization of above-market rate mortgages	0	0	$ 800,000
Identified intangible assets (included in other assets):
Gross amount	255,198,000	266,540,000
Accumulated amortization	(228,223,000)	(241,040,000)
Net	26,975,000	25,500,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	282,048,000	276,245,000
Accumulated amortization	(249,514,000)	(253,767,000)
Net	32,534,000	22,478,000
Net intangible assets reclassed to assets held for sale	0	0
Net intangible liabilities reclassed to liabilities relates to assets held for sale	0	$ 0
Other Identifiable Assets
Estimated annual amortization
2020	5,183,000
2021	3,638,000
2022	2,064,000
2023	1,435,000
2024	1,040,000
Acquired above-market leases, net of acquired below-market leases
Below Market Lease, Net, Amortization Income, Fiscal Year Maturity [Abstract]
2020	(4,320,000)
2021	(2,748,000)
2022	(2,224,000)
2023	(1,301,000)
2024	$ (834,000)
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
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
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
Right of use assets - financing leases | Maximum
Real Estate [Line Items]
Estimated useful life (in years)	40 years
Furniture and fixtures | Minimum
Real Estate [Line Items]
Estimated useful life (in years)	4 years
Furniture and fixtures | Maximum
Real Estate [Line Items]
Estimated useful life (in years)	7 years
Building | Minimum
Real Estate [Line Items]
Estimated useful life (in years)	3 years
Building | Maximum
Real Estate [Line Items]
Estimated useful life (in years)	40 years