Real Estate Intangibles (Summary of Real Estate Intangibles, excluding those classified as Held for Sale) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Assets
Gross historical cost	$ 1,581,159	$ 1,502,471
Accumulated amortization	(1,197,336)	(1,125,437)
Totals	$ 383,823	$ 377,034
Weighted-average amortization period in years	16 years	15 years 1 month 6 days
Liabilities:
Gross historical cost	$ 90,216	$ 68,970
Accumulated amortization	(44,266)	(39,629)
Totals	$ 45,950	$ 29,341
Weighted-average amortization period in years	14 years 8 months 12 days	20 years 1 month 6 days
In place lease intangibles
Assets
Gross historical cost	$ 1,410,725	$ 1,352,139
Above market tenant leases
Assets
Gross historical cost	63,935	58,443
Liabilities:
Gross historical cost	8,540	8,540
Below market ground leases
Assets
Gross historical cost	64,513	58,784
Lease commissions
Assets
Gross historical cost	41,986	33,105
Below market tenant leases
Liabilities:
Gross historical cost	$ 81,676	$ 60,430

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

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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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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274

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-Name Accounting Standards Codification
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-Paragraph 2
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Real estate intangible liabilities weighted average useful life.

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