Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Investment in Commercial Real Estate Properties
Rental revenue from amortization of acquired leases	$ 1,900,000	$ 1,200,000
Identified intangible assets (included in other assets):
Gross amount	239,562,000		$ 215,673,000
Accumulated amortization	(204,386,000)		(190,523,000)
Net	35,176,000		25,150,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	242,334,000		241,409,000
Accumulated amortization	(233,766,000)		(230,479,000)
Net	8,568,000		10,930,000
Net intangible assets reclassified to assets held for sale	0		0
Net intangible liabilities reclassified to liabilities relates to assets held for sale	$ 0		$ 0
Minimum | Above-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)	1 year
Minimum | Below-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)	1 year
Minimum | In-place leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)	1 year
Maximum | Above-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)	15 years
Maximum | Below-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)	15 years
Maximum | In-place leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)	15 years
Building | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)	3 years
Building | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)	40 years