Impact of the COVID-19 Pandemic - Narrative (Details) employee in Thousands, $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended	9 Months Ended	10 Months Ended	12 Months Ended
	Jan. 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) aircraft employee	Dec. 31, 2020 USD ($) aircraft employee	Dec. 31, 2020 USD ($) aircraft employee	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 employee
Impact Of Global Pandemic [Line Items]
Year-over-year decrease in system capacity (percent)					60.00%	50.00%
Number of aircraft retired | aircraft						227
Number of aircraft temporarily parked | aircraft				125	125	125
Number of aircraft removed from active service | aircraft						350
Number of employees volunteering for leave | employee						50
Salary reduction for CEO (percent)				100.00%
Salary reduction for company officers (percent)				50.00%
Work hours reduction for management and most front-line employee work groups (percent)				25.00%
Cash, cash equivalents, short-term investments, and amounts available under revolving credit facilities				$ 16,700	$ 16,700	$ 16,700
Financing transactions aggregate principal						25,900
Proceeds from payroll support program, CARES Act						5,600
Reduction in planned capital expenditures for the year						2,800
Capital expenditures						1,899	$ 4,936	$ 5,168
Subsequent event
Impact Of Global Pandemic [Line Items]
Proceeds from payroll support program	$ 1,400
Retired aircraft
Impact Of Global Pandemic [Line Items]
Cumulative net book value of retired aircraft				$ 500	$ 500	$ 500
Minimum
Impact Of Global Pandemic [Line Items]
Voluntary unpaid leave duration						30 days
Maximum
Impact Of Global Pandemic [Line Items]
Voluntary unpaid leave duration						12 months
Voluntary early retirement and separation programs
Impact Of Global Pandemic [Line Items]
Number of employees participating | employee				18	18	18			18
Forecast
Impact Of Global Pandemic [Line Items]
Proceeds from payroll support program		$ 1,500	$ 2,900
Forecast | Minimum
Impact Of Global Pandemic [Line Items]
Year-over-year decrease in system capacity (percent)			30.00%
Forecast | Maximum
Impact Of Global Pandemic [Line Items]
Year-over-year decrease in system capacity (percent)			40.00%
International
Impact Of Global Pandemic [Line Items]
Year-over-year decrease in system capacity (percent)					75.00%	65.00%
Domestic
Impact Of Global Pandemic [Line Items]
Year-over-year decrease in system capacity (percent)					50.00%	45.00%