Real Estate Intangibles (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Intangible Assets:
Gross historical cost	$ 1,548,621	$ 1,502,471
Accumulated amortization	(1,149,223)	(1,125,437)
Net book value	$ 399,398	$ 377,034
Weighted-average amortization period in years	14 years 5 months	15 years 1 month
Intangible Liabilities:
Gross historical cost	$ 80,246	$ 68,970
Accumulated amortization	(40,400)	(39,629)
Net book value	$ 39,846	$ 29,341
Weighted-average amortization period in years	15 years 12 months	20 years 1 month
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	$ 1,395,920	$ 1,352,139
Above market tenant leases [Member]
Intangible Assets:
Gross historical cost	58,402	58,443
Below market ground leases [Member]
Intangible Assets:
Gross historical cost	59,030	58,784
Lease commissions [Member]
Intangible Assets:
Gross historical cost	35,269	33,105
Below market tenant leases [Member]
Intangible Liabilities:
Gross historical cost	71,706	60,430
Above market ground leases [Member]
Intangible Liabilities:
Gross historical cost	$ 8,540	$ 8,540