Real Estate Intangibles - Summary of Real Estate Intangibles Excluding Those Classified as Held For Sale (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Assets:
Gross historical cost	$ 1,506,823	$ 1,500,030
Accumulated amortization	(1,204,529)	(1,177,513)
Net book value	$ 302,294	$ 322,517
Weighted-average amortization period in years	10 years 8 months 12 days	10 years 6 months
Liabilities:
Below market tenant leases	$ 75,451	$ 77,851
Accumulated amortization	(40,542)	(40,871)
Net book value	$ 34,909	$ 36,980
Weighted-average amortization period in years	8 years 2 months 12 days	8 years 3 months 18 days
In place lease intangibles
Assets:
Gross historical cost	$ 1,411,060	$ 1,406,705
Above market tenant leases
Assets:
Gross historical cost	53,066	52,621
Lease commissions
Assets:
Gross historical cost	$ 42,697	$ 40,704

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 of accumulated 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 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.

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-Publisher FASB
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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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