REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Deferred Revenue Arrangement [Line Items]
Receivable	$ 280.0		$ 280.0		$ 323.0
Costs and Expenses	709.0	$ 1,924.0	2,666.0	$ 3,775.0
COVID-19 Related Credits Issued			695.0
COVID-19 Credits Issued, Balance	568.0		568.0
Deferred Revenue, Refund Payments			363.0
Mileage Plan Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Receivable	$ 42.0		42.0		$ 105.0
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract liability revenue recognized			(21.0)	(51.0)
Passenger Revenue [Member]
Deferred Revenue Arrangement [Line Items]
Contract liability revenue recognized			$ 484.0	$ 563.0