Summary Of Significant Accounting Policies - Deferred Recertification And Dry Dock Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Deferred recertification and dry dock costs, net	$ 21,464	$ 16,065
Deferred recertification and dry dock costs accumulated amortization	21,800	15,700
Recertification and dry dock amortization expense	$ 14,300	$ 12,400	$ 8,300
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