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, 2020	Dec. 31, 2019
Assets:
Gross historical cost	$ 1,629,662	$ 1,617,051
Accumulated amortization	(1,195,330)	(1,181,158)
Net book value	$ 434,332	$ 435,893
Weighted-average amortization period in years	10 years 4 months 24 days	10 years 3 months 18 days
Liabilities:
Below market tenant leases	$ 86,233	$ 99,035
Accumulated amortization	(40,354)	(49,390)
Net book value	$ 45,879	$ 49,645
Weighted-average amortization period in years	8 years 7 months 6 days	8 years 7 months 6 days
In place lease intangibles
Assets:
Gross historical cost	$ 1,523,390	$ 1,513,836
Above market tenant leases
Assets:
Gross historical cost	60,484	59,540
Lease commissions
Assets:
Gross historical cost	$ 45,788	$ 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 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.

+ References

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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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