Credits issued to travelers stemming from the COVID-19 pandemic, net of estimated breakage

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REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Deferred Revenue Arrangement [Line Items]
Receivable	$ 279.0		$ 323.0
Costs and Expenses	1,957.0	$ 1,851.0
COVID-19 Related Credits Issued	402.0
Mileage Plan Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Receivable	88.0		$ 105.0
Mileage Plan Revenue [Member] \| Mileage plan other revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract liability revenue recognized	(22.0)	(28.0)
Passenger Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract liability revenue recognized	$ 489.0	$ 474.0