Real Estate Intangibles (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Assets
Other Finite-Lived Intangible Assets, Gross	$ 1,135,936,000	$ 1,070,754,000
Accumulated amortization	(776,501,000)	(571,008,000)
Net book value	359,435,000	499,746,000
Weighted-average amortization period in years	17 years 8 months	16 years 8 months
Liabilities
Real estate intangible liabilities, Gross	99,027,000	85,871,000
Accumulated amortization	(40,891,000)	(34,434,000)
Net book value	58,136,000	51,437,000
Weighted-average amortization period in years	14 years 5 months	14 years 4 months
In place lease intangibles [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	988,290,000	937,357,000
Above market tenant leases [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	65,684,000	55,939,000
Below market ground leases [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	62,426,000	59,165,000
Lease commissions [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	19,536,000	18,293,000
Gross historical cost [Member]
Assets
Other Finite-Lived Intangible Assets, Gross	1,135,936,000	1,070,754,000
Below market tenant leases [Member]
Liabilities
Real estate intangible liabilities, Gross	91,168,000	76,381,000
Above market ground leases [Member]
Liabilities
Real estate intangible liabilities, Gross	$ 7,859,000	$ 9,490,000