Real Estate Intangibles (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Assets
Gross historical cost	$ 1,502,471	$ 1,402,884
Accumulated amortization	(1,125,437)	(966,714)
Net book value	$ 377,034	$ 436,170
Weighted-average amortization period in years	15 years 1 month	13 years 8 months
Liabilities
Gross historical cost	$ 68,970	$ 97,575
Accumulated amortization	(39,629)	(52,134)
Net book value	$ 29,341	$ 45,441
Weighted-average amortization period in years	20 years 1 month	15 years 2 months
In place lease intangibles [Member]
Assets
Gross historical cost	$ 1,352,139	$ 1,252,143
Above market tenant leases [Member]
Assets
Gross historical cost	58,443	61,700
Below market ground leases [Member]
Assets
Gross historical cost	58,784	61,628
Lease commissions [Member]
Assets
Gross historical cost	33,105	27,413
Gross historical cost [Member]
Assets
Gross historical cost	1,502,471	1,402,884
Below market tenant leases [Member]
Liabilities
Gross historical cost	60,430	89,468
Above market ground leases [Member]
Liabilities
Gross historical cost	$ 8,540	$ 8,107

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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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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