Real Estate Intangibles (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Assets
Other Finite-Lived Intangible Assets, Gross	$ 1,350,585	$ 1,135,936
Accumulated amortization	$ (881,096)	$ (776,501)
Weighted-average amortization period in years	13 years 5 months	17 years 8 months
Liabilities
Real estate intangible liabilities, Gross	$ 100,996	$ 99,027
Accumulated amortization	(46,048)	(40,891)
Net book value	$ 54,948	$ 58,136
Weighted-average amortization period in years	14 years 6 months	14 years 5 months
In place lease intangibles [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	$ 1,179,537	$ 988,290
Above market tenant leases [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	67,529	65,684
Below market ground leases [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	80,224	62,426
Lease commissions [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	23,295	19,536
Gross historical cost [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	1,350,585	1,135,936
Below market tenant leases [Member]
Liabilities
Real estate intangible liabilities, Gross	93,089	91,168
Above market ground leases [Member]
Liabilities
Real estate intangible liabilities, Gross	$ 7,907	$ 7,859