Real Estate Intangibles (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Intangible Assets:
Gross historical cost	$ 1,187,094,000	$ 1,135,936,000
Accumulated amortization	789,546,000	776,501,000
Net book value	397,548,000	359,435,000
Weighted-average amortization period in years	16 years 2 months	17 years 8 months
Intangible Liabilities:
Gross historical cost	99,224,000	99,027,000
Accumulated amortization	42,853,000	40,891,000
Net book value	56,371,000	58,136,000
Weighted-average amortization period in years	14 years 5 months	14 years 5 months
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	1,037,697,000	988,290,000
Above market tenant leases [Member]
Intangible Assets:
Gross historical cost	66,162,000	65,684,000
Below market ground leases [Member]
Intangible Assets:
Gross historical cost	62,426,000	62,426,000
Lease commissions [Member]
Intangible Assets:
Gross historical cost	20,809,000	19,536,000
Below market tenant leases [Member]
Intangible Liabilities:
Gross historical cost	91,365,000	91,168,000
Above market ground leases [Member]
Intangible Liabilities:
Gross historical cost	$ 7,859,000	$ 7,859,000