Real Estate Intangibles (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets and Liabilities [Line Items]
Finite-Lived Intangible Assets, Gross	$ 1,534,755	$ 1,502,471
Finite-Lived Intangible Assets, Accumulated Amortization	(1,150,396)	(1,125,437)
Net book value	$ 384,359	$ 377,034
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	14 years 7 months 28 days	15 years 1 month 6 days
Real Estate Intangible Liabilities Gross	$ 80,200	$ 68,970
Real Estate Intangible Liabilities Accumulated Amortization	(41,503)	(39,629)
Real Estate Intangible Liabilities Net	$ 38,697	$ 29,341
Real Estate Intangible Liabilities Weighted Average Useful Life	16 years 7 days	20 years 1 month 6 days
In place lease intangibles [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Finite-Lived Intangible Assets, Gross	$ 1,380,504	$ 1,352,139
Above market tenant leases [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Finite-Lived Intangible Assets, Gross	58,373	58,443
Real Estate Intangible Liabilities Gross	8,540	8,540
Below market ground leases [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Finite-Lived Intangible Assets, Gross	59,030	58,784
Lease commissions [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Finite-Lived Intangible Assets, Gross	36,848	33,105
Below market tenant leases [Member]
Finite-Lived Intangible Assets and Liabilities [Line Items]
Real Estate Intangible Liabilities Gross	$ 71,660	$ 60,430