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Long-Term Debt (Detail)			1 Months Ended		6 Months Ended		12 Months Ended			60 Months Ended
Long-Term Debt (Detail)	Apr. 20, 2020	Feb. 13, 2020 USD ($)	Jul. 31, 2019 USD ($)	Feb. 28, 2019 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2020 USD ($) Aircraft	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 20, 2030	Oct. 31, 2020 USD ($)	Apr. 30, 2020 USD ($) Aircraft	Mar. 31, 2020 USD ($)	Oct. 31, 2019 USD ($) Aircraft	Mar. 31, 2019 USD ($) Aircraft	Feb. 01, 2019 USD ($)	Sep. 30, 2018 USD ($) Aircraft
Consolidated Variable Interest Entity
Debt, Weighted Average Interest Rate														3.20%	3.80%		4.00%
PP&E from VIEs																	$ 37,800,000
Consolidated Variable Interest Entity \| Variable Interest Entity, Primary Beneficiary [Member]
Liabilities														$ 23,500,000	$ 44,000,000.0		$ 44,000,000.0
PP&E from VIEs														$ 18,600,000	$ 38,500,000
Consolidated Variable Interest Entity \| Airbus A320 Aircraft Series
Number Of Aircrafts Included As Collateral \| Aircraft														1	1		1
Senior Secured Revolving Credit Facility
Debt Instrument, Face Amount											$ 150,000,000.0
Line of Credit Facility, Maximum Borrowing Capacity						$ 81,000,000.0
Line of Credit Facility, Fair Value of Amount Outstanding								$ 81,000,000.0
Debt Instrument, Term						24 months
Line of Credit Facility, Periodic Payment, Principal							$ 27,100,000
Line of Credit Facility, Remaining Borrowing Capacity							$ 53,900,000
Unsecured Debt
Repayments of Unsecured Debt				$ 347,900,000
Unsecured Debt \| Payroll Support Program Loan CARES Act [Member]
Debt Instrument, Interest Rate, Stated Percentage							1.00%
Proceeds From Payroll Support Program, Low-Interest Loan					$ 23,100,000
Debt Instrument, Face Amount		$ 545,500,000			84,000,000.0							$ 31,000,000.0				$ 450,000,000.0
Debt Instrument, Increase (Decrease), Net		$ 100,000,000.0
Total long-term debt and finance lease obligations, net of related costs							$ 1,659,011,000	1,421,853,000
Long-term Debt, Current Maturities							217,234,000	173,274,000
Long-term Debt, Excluding Current Maturities							1,441,777,000	1,248,579,000
Number Of Aircrafts Included As Collateral \| Aircraft												2
Repayments of Unsecured Debt			$ 102,100,000
Line of Credit Facility, Maximum Borrowing Capacity													$ 175,000,000.0
Line of Credit Facility, Fair Value of Amount Outstanding							0
Debt Instrument, Term	10 years
Debt Instrument, Periodic Payment, Principal				1,400,000
Gain (Loss) on Extinguishment of Debt				$ (3,700,000)			$ (1,222,000)	(3,677,000)	$ 0
Number of Terms to be Included in the Collateral Pool						2 years
Capital Leased Assets, Number of Units \| Aircraft							6
Liabilities							$ 2,559,562,000	2,127,252,000
PP&E from VIEs							3,258,925,000	3,010,803,000
Termination Loans							19,800,000
Proceeds From Payroll Support Program, Low-Interest Loan					$ 23,100,000
Variable Interest Entity, Primary Beneficiary [Member]
PP&E from VIEs							187,166,000	195,796,000
Fixed Rate
Notes Payable							$ 525,240,000	$ 235,071,000
Long-term Debt, Weighted Average Interest Rate, at Point in Time							5.70%	3.70%
Variable Rate
Notes Payable							$ 1,133,771,000	$ 1,186,782,000
Long-term Debt, Weighted Average Interest Rate, at Point in Time							2.40%	4.50%
Payroll Support Program Loan CARES Act [Member] \| Forecast [Member] \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]
Debt Instrument, Basis Spread on Variable Rate										2.00%
Senior Secured Notes due 2024 [Member]
Debt Instrument, Interest Rate, Stated Percentage							8.50%
Senior Secured Revolving Credit Facility
Number Of Aircrafts Included As Collateral \| Aircraft							6
Long-term Debt
reduction in debt interest rate		15000.00%