Real Estate Intangibles (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Intangible Assets:
Gross historical cost	$ 1,421,277	$ 1,402,884
Accumulated amortization	1,010,364	966,714
Net book value	$ 410,913	$ 436,170
Weighted-average amortization period in years	14 years 5 months	13 years 8 months
Intangible Liabilities:
Gross historical cost	$ 98,852	$ 97,575
Accumulated amortization	54,354	52,134
Net book value	$ 44,498	$ 45,441
Weighted-average amortization period in years	15 years 1 month	15 years 2 months
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	$ 1,267,280	$ 1,252,143
Above market tenant leases [Member]
Intangible Assets:
Gross historical cost	63,184	61,700
Below market ground leases [Member]
Intangible Assets:
Gross historical cost	62,224	61,628
Lease commissions [Member]
Intangible Assets:
Gross historical cost	28,589	27,413
Below market tenant leases [Member]
Intangible Liabilities:
Gross historical cost	90,745	89,468
Above market ground leases [Member]
Intangible Liabilities:
Gross historical cost	$ 8,107	$ 8,107