Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Real Estate [Line Items]
Weighted average amortization period, above-market lease costs	6 years 3 months 18 days
Weighted average amortization period, below-market lease costs	5 years 7 months 6 days
Weighted average amortization period, in-place lease costs	5 years 3 months 18 days
Period from cessation of major construction activity to completion of construction project, maximum	1 year
Depreciation expense (including amortization of the capital lease asset)	$ 277,500,000	$ 233,500,000	$ 242,800,000
Amortization of above and below Market Leases	5,900,000	4,500,000	$ 6,800,000
Identified intangible assets (included in other assets):
Gross amount	215,673,000	255,198,000
Accumulated amortization	(190,523,000)	(228,223,000)
Net	25,150,000	26,975,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	241,409,000	282,048,000
Accumulated amortization	(230,479,000)	(249,514,000)
Net	10,930,000	32,534,000
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
2021	4,899,000
2022	3,456,000
2023	2,841,000
2024	2,520,000
2025	1,427,000
Acquired above-market leases, net of acquired below-market leases
Below Market Lease, Net, Amortization Income, Fiscal Year Maturity [Abstract]
2021	(1,403,000)
2022	(119,000)
2023	91,000
2024	258,000
2025	$ 781,000
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