Real Estate Intangibles (Summary of Real Estate Intangibles Excluding those Classified as Held For Sale) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Assets
Gross historical cost	$ 1,617,051	$ 1,581,159
Accumulated amortization	(1,181,158)	(1,197,336)
Totals	$ 435,893	$ 383,823
Weighted-average amortization period in years	10 years 3 months 18 days	16 years
Liabilities:
Gross historical cost	$ 99,035	$ 90,216
Accumulated amortization	(49,390)	(44,266)
Totals	$ 49,645	$ 45,950
Weighted-average amortization period in years	8 years 7 months 6 days	14 years 8 months 12 days
In place lease intangibles
Assets
Gross historical cost	$ 1,513,836	$ 1,410,725
Below market tenant leases
Liabilities:
Gross historical cost	99,035	81,676
Above market tenant leases
Assets
Gross historical cost	59,540	63,935
Liabilities:
Gross historical cost	0	8,540
Below market ground leases
Assets
Gross historical cost	0	64,513
Lease commissions
Assets
Gross historical cost	$ 43,675	$ 41,986

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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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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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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