Commitments and Contingencies (Commitments) (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Long-term Purchase Commitment [Line Items]
aircraft purchase deposit	$ 19,900			$ 19,900
Long-term Purchase Commitment, Amount				195,400
total government grant awarded	75,800			75,800
Income Tax (Expense) Benefit, Continuing Operations, Government Grants	15,700	$ 21,700	$ 9,800	47,231	$ 0	$ 0
unrecognized government grants received	$ 28,500			$ 28,500