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

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Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Investment in Commercial Real Estate Properties
Rental revenue from amortization of acquired leases	$ 1,200	$ 1,800	$ 2,400	$ 4,200
Identified intangible assets (included in other assets):
Gross amount	282,932		282,932		$ 266,540
Accumulated amortization	(247,418)		(247,418)		(241,040)
Net	35,514		35,514		25,500
Identified intangible liabilities (included in deferred revenue):
Gross amount	311,091		311,091		276,245
Accumulated amortization	(258,222)		(258,222)		(253,767)
Net	$ 52,869		$ 52,869		$ 22,478
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