Real Estate Intangibles (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Intangible Assets:
Gross historical cost	$ 1,492,279	$ 1,402,884
Accumulated amortization	(1,098,814)	(966,714)
Net book value	$ 393,465	$ 436,170
Weighted-average amortization period in years	14 years 6 months	13 years 8 months
Intangible Liabilities:
Gross historical cost	$ 99,211	$ 97,575
Accumulated amortization	(57,449)	(52,134)
Net book value	$ 41,762	$ 45,441
Weighted-average amortization period in years	15 years 6 months	15 years 2 months
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	$ 1,333,368	$ 1,252,143
Above market tenant leases [Member]
Intangible Assets:
Gross historical cost	64,408	61,700
Below market ground leases [Member]
Intangible Assets:
Gross historical cost	62,224	61,628
Lease commissions [Member]
Intangible Assets:
Gross historical cost	32,279	27,413
Below market tenant leases [Member]
Intangible Liabilities:
Gross historical cost	90,671	89,468
Above market ground leases [Member]
Intangible Liabilities:
Gross historical cost	$ 8,540	$ 8,107