Summary Of Significant Accounting Policies - Recertification Costs And Deferred Dry Dock Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Deferred dry dock costs, net	$ 12,368	$ 14,766
Deferred dry dock costs accumulated amortization	7,100	10,700
Dry dock amortization expense	$ 6,900	$ 14,000	$ 10,800
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Dry dock amortization period	30 months
Dry dock recertification process period	1 month
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Dry dock amortization period	60 months
Dry dock recertification process period	2 months