Real Estate Intangibles (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Intangible Assets:
Gross historical cost	$ 1,345,424	$ 1,350,585
Accumulated amortization	949,727	881,096
Net book value	$ 395,697	$ 469,489
Weighted-average amortization period in years	15 years 10 months	13 years 5 months
Intangible Liabilities:
Gross historical cost	$ 100,962	$ 100,996
Accumulated amortization	49,874	46,048
Net book value	$ 51,088	$ 54,948
Weighted-average amortization period in years	15 years	14 years 6 months
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	$ 1,194,925	$ 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	62,447	80,224
Lease commissions [Member]
Intangible Assets:
Gross historical cost	25,508	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