Basis of Presentation and Accounting Policies Deferred Gains (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Deferred Gains	$ 64,715	$ 81,691	$ 72,453	$ 82,423
Increase (Decrease) in Deferred Charges	0	7,720
Operating Expense [Member]
Amortization of Deferred Gains	5,039	7,242
Gain on Asset Dispositions and Impairments, Net [Member]
Amortization of Deferred Gains	1,012	1,210
Sales Leaseback and Financed Equipment Sales [Member]
Reductions of Deferred Gains on Repurchased Vessels and Other	$ 1,687	$ 0