Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Jun. 30, 2020	Dec. 31, 2019
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		$ 1	$ 10
Gross asset amounts offset in the Balance Sheet		(1)	(10)
Liability derivative contracts, net		0	0
Fuel derivatives | Other Assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		31	15
Gross asset amounts offset in the Balance Sheet		(31)	(15)
Liability derivative contracts, net	[1]	0	0
Interest rate derivatives | Accrued Liabilities [Member]
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		60	5
Gross asset amounts offset in the Balance Sheet		0	0
Liability derivative contracts, net	[1]	60	5
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		14	1
Gross asset amounts offset in the Balance Sheet		0	0
Liability derivative contracts, net		$ 14	$ 1

[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.