Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Investment in Commercial Real Estate Properties
Rental revenue from amortization of acquired leases	$ 1,200,000	$ 1,200,000
Identified intangible assets (included in other assets):
Gross amount	225,025,000		$ 255,198,000
Accumulated amortization	(202,581,000)		(228,223,000)
Net	22,444,000		26,975,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	274,208,000		282,048,000
Accumulated amortization	(246,464,000)		(249,514,000)
Net	27,744,000		32,534,000
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)	14 years
Maximum | Below-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)	14 years
Maximum | In-place leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)	14 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