Summary Of Significant Accounting Policies - Deferred Recertification And Dry Dock Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Deferred recertification and dry dock costs, net	$ 8,525	$ 12,769
Deferred recertification and dry dock costs accumulated amortization	15,400	7,300
Recertification and dry dock amortization expense	$ 8,300	$ 7,000	$ 14,000
Minimum
Finite-Lived Intangible Assets [Line Items]
Recertification and dry dock amortization period	30 months
Recertification process period	1 month
Maximum
Finite-Lived Intangible Assets [Line Items]
Recertification and dry dock amortization period	60 months
Recertification process period	3 months