Debt and Other Obligations - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Debt Instrument [Line Items]
Commitment and other fees	$ 604	$ 228	$ 1,032	$ 445
Amortization of deferred financing costs	3,274	2,526	6,783	4,632
Total	39,662	27,792	84,468	51,670
Accretion of 8.00% senior secured notes and convertible debt			753	2,029
8.00% senior secured notes due 2025
Debt Instrument [Line Items]
Interest expense				0
Accretion of 8.00% senior secured notes and convertible debt	300	2,000	800	2,000
Interest expense	13,200	1,100	30,200	1,100
8.00% senior secured notes due 2025 | 8.00% Senior Secured Notes Due 2025
Debt Instrument [Line Items]
Interest expense	13,541	0	30,975
Fixed-rate term loans
Debt Instrument [Line Items]
Interest expense	10,746	10,811	21,649	21,237
Unsecured term loans | Payroll Support Program, CARES Act
Debt Instrument [Line Items]
Interest expense	282	57	481	57
2015-1 EETC Class A
Debt Instrument [Line Items]
Interest expense	3,178	3,403	6,453	6,952
2015-1 EETC Class B
Debt Instrument [Line Items]
Interest expense	664	752	1,369	1,548
2015-1 EETC Class C
Debt Instrument [Line Items]
Interest expense	997	1,138	2,053	2,347
2017-1 EETC Class AA
Debt Instrument [Line Items]
Interest expense	1,759	1,868	3,514	3,755
2017-1 EETC Class A
Debt Instrument [Line Items]
Interest expense	634	673	1,267	1,354
2017-1 EETC Class B
Debt Instrument [Line Items]
Interest expense	556	623	1,113	1,267
2017-1 EETC Class C
Debt Instrument [Line Items]
Interest expense	1,092	1,092	2,171	2,184
Convertible notes
Debt Instrument [Line Items]
Interest expense	1,749	3,138	3,827	3,138
Revolving credit facilities
Debt Instrument [Line Items]
Interest expense	562	1,456	1,733	2,614
Finance leases
Debt Instrument [Line Items]
Interest expense	$ 24	$ 27	$ 48	$ 140