Schedule III - Schedule of Real Estate and Accumulated Depreciation (Reconciliation of Real Estate Accumulated Depreciation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule III - Schedule of Real Estate and Accumulated Depreciation [Abstract]
Gross amount of accumulated depreciation at beginning	$ (2,396,587)	$ (2,160,530)	$ (1,912,906)
Depreciation	(248,818)	(254,982)	(282,831)
Other		(5,557)
Total Additions	(248,818)	(260,539)	(282,831)
Amount of accumulated depreciation for assets sold or disposed	17,051	24,483	25,909
Other	513		9,298
Total deductions	17,564	24,483	35,207
Balance at end	$ (2,627,841)	$ (2,396,587)	$ (2,160,530)