Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Offsetting Assets [Line Items]
Asset derivative contracts, net		$ 77	$ 49
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		6	48
Gross liability amounts offset in the Balance Sheet		(1)	(10)
Asset derivative contracts, net		5	38
Fuel derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		103	62
Gross liability amounts offset in the Balance Sheet		(26)	(15)
Asset derivative contracts, net	[1]	77	47
Interest rate derivatives | Other Assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets		0	2
Gross liability amounts offset in the Balance Sheet		0	0
Asset derivative contracts, net	[1]	$ 0	$ 2

[1] The net amounts of derivative assets and liabilities are reconciled to the individual line item amounts presented in the unaudited Condensed Consolidated Balance Sheet in Note 10.