Real Estate Intangibles (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets and Liabilities [Line Items]
Assets - gross historical cost	$ 1,562,650	$ 1,502,471
Assets - accumulated amortization	(1,190,035)	(1,125,437)
Net book value	$ 372,615	$ 377,034
Assets - weighted-average amortization period in years	16 years 11 days	15 years 1 month 6 days
Liabilities - gross historical cost	$ 80,106	$ 68,970
Liabilities - accumulated Amortization	(42,834)	(39,629)
Real Estate Intangible Liabilities Net	$ 37,272	$ 29,341
Liabilities - weighted-average amortization period in years	16 years 1 month 16 days	20 years 1 month 6 days
In place lease intangibles [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Assets - gross historical cost	$ 1,398,850	$ 1,352,139
Above market tenant leases [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Assets - gross historical cost	59,011	58,443
Liabilities - gross historical cost	8,540	8,540
Below market ground leases [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Assets - gross historical cost	65,022	58,784
Lease commissions [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Assets - gross historical cost	39,767	33,105
Below market tenant leases [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Liabilities - gross historical cost	$ 71,566	$ 60,430