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, 2019	Dec. 31, 2018
Assets:
Gross historical cost	$ 1,539,363	$ 1,581,159
Accumulated amortization	(1,214,735)	(1,197,336)
Net book value	$ 324,628	$ 383,823
Weighted-average amortization period in years	10 years 1 month 6 days	16 years
Liabilities:
Gross historical cost	$ 82,981	$ 90,216
Accumulated amortization	(44,580)	(44,266)
Net book value	$ 38,401	$ 45,950
Weighted-average amortization period in years	8 years 8 months 12 days	14 years 8 months 12 days
In place lease intangibles
Assets:
Gross historical cost	$ 1,430,342	$ 1,410,725
Above market tenant leases
Assets:
Gross historical cost	64,684	63,935
Liabilities:
Gross historical cost		8,540
Below market ground leases
Assets:
Gross historical cost		64,513
Lease commissions
Assets:
Gross historical cost	44,337	41,986
Below market tenant leases
Liabilities:
Gross historical cost	$ 82,981	$ 81,676

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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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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