Real Estate Intangibles (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Intangible Assets:
Gross historical cost	$ 1,423,032	$ 1,350,585
Accumulated amortization	971,883	881,096
Net book value	$ 451,149	$ 469,489
Weighted-average amortization period in years	14 years	13 years 5 months
Intangible Liabilities:
Gross historical cost	$ 101,614	$ 100,996
Accumulated amortization	51,727	46,048
Net book value	$ 49,887	$ 54,948
Weighted-average amortization period in years	15 years 5 months	14 years 6 months
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	$ 1,272,177	$ 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	62,257	80,224
Lease commissions [Member]
Intangible Assets:
Gross historical cost	26,054	23,295
Below market tenant leases [Member]
Intangible Liabilities:
Gross historical cost	93,507	93,089
Above market ground leases [Member]
Intangible Liabilities:
Gross historical cost	$ 8,107	$ 7,907