Debt and Other Obligations - Narrative (Details)					1 Months Ended	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 17, 2020 USD ($)	May 12, 2020 USD ($) $ / shares shares	Apr. 20, 2020 USD ($)	Mar. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) aircraft	Sep. 30, 2020 USD ($) aircraft	Jul. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) aircraft	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2021 aircraft	Mar. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Long-term debt					$ 3,506,400,000	$ 3,506,400,000				$ 3,506,400,000		$ 2,213,600,000
Long-term debt					3,584,800,000	3,584,800,000				3,584,800,000
Equity component value of convertible debt issuance, net								$ 55,590,000
Repayments of long-term debt						58,600,000			$ 40,100,000	208,000,000.0	$ 125,900,000
Accretion of convertible debt and 8.00% senior secured notes										5,898,000	$ 0
Debt discount					60,000,000.0	60,000,000.0				60,000,000.0		40,400,000
Fixed-rate term loans
Debt Instrument [Line Items]
Principal amount					291,000,000.0	291,000,000.0				291,000,000.0
Long-term debt					1,014,500,000	1,014,500,000				1,014,500,000		778,200,000
Notes payable To United States Treasury | Payroll Support Program, Low-Interest Loan
Debt Instrument [Line Items]
Principal amount			$ 70,400,000		$ 2,900,000	2,900,000	$ 70,400,000			2,900,000
Term			10 years		10 years		10 years
Notes payable To United States Treasury | Payroll Support Program
Debt Instrument [Line Items]
Principal amount							$ 9,700,000
Stated interest rate			1.00%
Notes payable To United States Treasury | Payroll Support Program | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate			2.00%
Unsecured term loans
Debt Instrument [Line Items]
Long-term debt												0
Unsecured term loans | Payroll Support Program
Debt Instrument [Line Items]
Long-term debt					$ 73,300,000	73,300,000				73,300,000
Convertible debt
Debt Instrument [Line Items]
Long-term debt					105,200,000	105,200,000				105,200,000		0
Accretion of convertible debt and 8.00% senior secured notes						3,800,000				5,800,000
Interest expense						2,100,000				3,200,000
Convertible debt | 4.75% Convertible Senior Notes
Debt Instrument [Line Items]
Principal amount		$ 175,000,000.0
Term		5 years
Stated interest rate		4.75%
Proceeds from issuance of convertible debt		$ 168,300,000
Payments of financing costs		$ 6,700,000
Long-term debt					95,600,000	95,600,000				95,600,000
Debt issuance costs					$ 3,800,000	3,800,000				3,800,000
Equity component value of convertible debt issuance, net						72,700,000
Common stock and warrants issuance costs										2,900,000
Debt instrument, convertible, if-converted value in excess of principal						$ 61,300,000				$ 53,100,000
Convertible debt, conversion price threshold		130.00%
Number of trading days required for conversion price threshold		20 days
Number of consecutive days required for trading days for conversion price threshold		30 days
Number of consecutive business days for conversion price threshold		5 days
Number of consecutive trading days measurement period		5 days
Principal amount portion for trading price threshold		$ 1,000
Trading price threshold per principal amount portion		98.00%
Shares conversion rate per portion of principal amount (in shares) | shares		78.4314
Conversion price (in dollars per share) | $ / shares		$ 12.75
8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Stated interest rate					8.00%	8.00%				8.00%
Long-term debt					$ 841,400,000	$ 841,400,000				$ 841,400,000		0
Accretion of convertible debt and 8.00% senior secured notes						100,000				100,000
Interest expense						$ 2,500,000				$ 2,500,000
8.00% senior secured notes due in 2025 | 8.00% Senior Secured Notes Due 2025
Debt Instrument [Line Items]
Principal amount	$ 850,000,000
Stated interest rate	8.00%				8.00%	8.00%				8.00%
Debt issuance costs					$ 17,400,000	$ 17,400,000				$ 17,400,000
Proceeds from Issuance of Secured Debt						823,900,000
Debt discount					$ 8,700,000	$ 8,700,000				$ 8,700,000
Debt instrument, first-priority security interest, equity interests percentage	100.00%
Debt instrument, redemption price, percentage	100.00%
Debt covenant, percent of principal subject to prepayment	100.00%
Debt covenant, percent of principal subject to prepayment upon change of control	101.00%
Debt covenant, maximum pre-paid miles allowed during fiscal year, value	$ 25,000,000.0
Debt covenant, minimum liquidity requirement	$ 400,000,000
A-320-Neo
Debt Instrument [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft					19	19				19
A-320-Neo | Forecast
Debt Instrument [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft													43
Fixed-rate term loans | Minimum
Debt Instrument [Line Items]
Long-term debt, term					11 years	11 years				11 years
Fixed-rate term loans | Maximum
Debt Instrument [Line Items]
Long-term debt, term					12 years	12 years				12 years
Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Debt covenant, amount of unrestricted cash and cash equivalents and unused commitments on all revolving credit facilities				$ 400,000,000
Debt covenant, maximum amount of unused commitments from 2022 revolving credit facility				$ 200,000,000
Debt covenant, collateral coverage ratio				1.0
Revolving Credit Facility | Line of Credit | 2022 Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				$ 110,000,000.0				$ 180,000,000.0						$ 110,000,000.0
Line of credit facility, optional maximum borrowing capacity subject to consent of lenders				$ 350,000,000
Proceeds from revolving credit facility										$ 180,000,000.0
Revolving Credit Facility | Line of Credit | Revolving credit facility due in 2021
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity														$ 111,200,000
Proceeds from revolving credit facility										$ 95,100,000
Revolving Credit Facility | Line of Credit | 2020 Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity															$ 160,000,000.0
Proceeds from revolving credit facility												$ 160,000,000.0
Revolving Credit Facility | Minimum | Line of Credit | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate				0.00%
Revolving Credit Facility | Minimum | Line of Credit | Certain Market Interest Rates
Debt Instrument [Line Items]
Basis spread on variable rate				0.00%
Revolving Credit Facility | Maximum | Line of Credit | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate				2.00%
Revolving Credit Facility | Maximum | Line of Credit | Certain Market Interest Rates
Debt Instrument [Line Items]
Basis spread on variable rate				1.00%
Airbus | Airbus A320 | Fixed-rate term loans
Debt Instrument [Line Items]
Number of delivered aircraft financed with debt | aircraft										7