Real Estate Intangibles (Summary of Real Estate Intangibles Excluding those Classified as Held For Sale) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Assets
Gross historical cost	$ 1,500,030	$ 1,617,051
Accumulated amortization	(1,177,513)	(1,181,158)
Net book value	$ 322,517	$ 435,893
Weighted-average amortization period in years	10 years 6 months	10 years 3 months 18 days
Liabilities:
Gross historical cost	$ 77,851	$ 99,035
Accumulated amortization	(40,871)	(49,390)
Net book value	$ 36,980	$ 49,645
Weighted-average amortization period in years	8 years 3 months 18 days	8 years 7 months 6 days
In place lease intangibles
Assets
Gross historical cost	$ 1,406,705	$ 1,513,836
Above market tenant leases
Assets
Gross historical cost	52,621	59,540
Lease commissions
Assets
Gross historical cost	$ 40,704	$ 43,675

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of 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 leases acquired as part of a real property acquisition at below market lease rate with a finite life.

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Amount after amortization of leases acquired as part of a real property acquisition at below market lease rate with a finite life.

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

+ References

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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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Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

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Real estate intangible liabilities weighted average useful life.

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