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Represents the accumulated amount of amortization of a major finite-lived intangible liability class.

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Net Intangible Assets Transferred to Assets Held for Sale

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Net Intangible Liabilities Transferred to Liabilities Related to Assets Held for Sale

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Investment in Commercial Real Estate Properties
Rental revenue from amortization of acquired leases	$ 1,600,000	$ 1,200,000	$ 2,900,000	$ 2,400,000
Identified intangible assets (included in other assets):
Gross amount	224,411,000		224,411,000		$ 255,198,000
Accumulated amortization	(205,314,000)		(205,314,000)		(228,223,000)
Net	19,097,000		19,097,000		26,975,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	273,240,000		273,240,000		282,048,000
Accumulated amortization	(250,912,000)		(250,912,000)		(249,514,000)
Net	22,328,000		22,328,000		32,534,000
Net intangible assets reclassified to assets held for sale	0		0		0
Net intangible liabilities reclassified to liabilities relates to assets held for sale	$ 400,000		$ 400,000		$ 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