Debt and Other Obligations - Narrative (Details)					2 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2020 USD ($)	May 12, 2020 USD ($) $ / shares shares	Apr. 20, 2020 USD ($)	Mar. 30, 2020 USD ($)	Jun. 30, 2020 USD ($) aircraft	Jun. 30, 2020 USD ($) aircraft	Jun. 30, 2020 USD ($) aircraft	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) aircraft	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2021 aircraft	Mar. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Proceeds from Payroll Support Program						$ 301,300,000
Maximum expected proceeds to be received from Payroll Support Program			$ 334,700,000
Long-term debt					$ 2,664,800,000	2,664,800,000	$ 2,664,800,000		$ 2,664,800,000		$ 2,213,600,000
Long-term debt					2,664,800,000	2,664,800,000	2,664,800,000		2,664,800,000
Equity component value of convertible debt issuance, net							55,590,000
Repayments of long-term debt							106,900,000	$ 47,900,000	149,400,000	$ 85,800,000
Accretion of convertible debt									2,029,000	$ 0
CARES Act
Debt Instrument [Line Items]
Payments of financing costs							500,000
Forecast
Debt Instrument [Line Items]
Proceeds from Payroll Support Program	$ 33,400,000
Payroll Support Program, Low-Interest Loan | Forecast
Debt Instrument [Line Items]
Term	10 years
Fixed-rate term loans
Debt Instrument [Line Items]
Principal amount					249,000,000.0	249,000,000.0	249,000,000.0		249,000,000.0
Long-term debt					992,200,000	992,200,000	992,200,000		992,200,000		778,200,000
Notes Payable To United States Treasury
Debt Instrument [Line Items]
Principal amount					60,400,000	$ 60,400,000	60,400,000		60,400,000
Term						10 years
Notes Payable To United States Treasury | Forecast
Debt Instrument [Line Items]
Principal amount	$ 10,000,000.0
Notes Payable To United States Treasury | Payroll Support Program, Low-Interest Loan
Debt Instrument [Line Items]
Principal amount			$ 70,400,000		60,400,000	$ 60,400,000	60,400,000		60,400,000
Term			10 years
Notes Payable To United States Treasury | Payroll Support Program Term Loan | Forecast
Debt Instrument [Line Items]
Principal amount	10,000,000.0
Notes Payable To United States Treasury | Payroll Support Program
Debt Instrument [Line Items]
Stated interest rate			1.00%
Notes Payable To United States Treasury | Payroll Support Program | Forecast
Debt Instrument [Line Items]
Principal amount	$ 70,400,000
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					60,400,000	60,400,000	60,400,000		60,400,000		0
Convertible debt
Debt Instrument [Line Items]
Long-term debt					101,400,000	101,400,000	101,400,000		101,400,000		0
Accretion of convertible debt									2,000,000.0
Interest expense							1,100,000
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		95,600,000
Debt issuance costs					3,800,000	$ 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					$ 38,000,000
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
A-320-Neo
Debt Instrument [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft					20	20	20		20
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		11 years
Fixed-rate term loans | Maximum
Debt Instrument [Line Items]
Long-term debt, term					12 years	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	$ 180,000,000.0	$ 180,000,000.0		$ 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	$ 111,200,000	$ 111,200,000		111,200,000
Proceeds from revolving credit facility									$ 111,200,000
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									6