Real Estate Intangibles (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Intangible Assets:
Gross historical cost	$ 1,346,064	$ 1,350,585
Accumulated amortization	923,712	881,096
Net book value	$ 422,352	$ 469,489
Weighted-average amortization period in years	14 years 7 months	13 years 5 months
Intangible Liabilities:
Gross historical cost	$ 100,962	$ 100,996
Accumulated amortization	47,983	46,048
Net book value	$ 52,979	$ 54,948
Weighted-average amortization period in years	14 years 9 months	14 years 6 months
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	$ 1,196,335	$ 1,179,537
Above market tenant leases [Member]
Intangible Assets:
Gross historical cost	62,544	67,529
Below market ground leases [Member]
Intangible Assets:
Gross historical cost	62,643	80,224
Lease commissions [Member]
Intangible Assets:
Gross historical cost	24,542	23,295
Below market tenant leases [Member]
Intangible Liabilities:
Gross historical cost	93,054	93,089
Above market ground leases [Member]
Intangible Liabilities:
Gross historical cost	$ 7,908	$ 7,907