Real Estate Intangibles - Summary of Real Estate Intangibles Excluding Those Classified as Held For Sale (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Assets:
Gross historical cost	$ 1,653,415	$ 1,500,030
Accumulated amortization	(1,249,032)	(1,177,513)
Net book value	$ 404,383	$ 322,517
Weighted-average amortization period in years	8 years 3 months 18 days	10 years 6 months
Liabilities:
Below market tenant leases	$ 75,083	$ 77,851
Accumulated amortization	(43,484)	(40,871)
Net book value	$ 31,599	$ 36,980
Weighted-average amortization period in years	8 years 4 months 24 days	8 years 3 months 18 days
In place lease intangibles
Assets:
Gross historical cost	$ 1,549,294	$ 1,406,705
Above market tenant leases
Assets:
Gross historical cost	54,542	52,621
Lease commissions
Assets:
Gross historical cost	$ 49,579	$ 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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