Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities
Gross amounts of recognized liabilities		$ 10	$ 0
Gross amounts offset in the Balance Sheet		(10)	0
Liabilities derivative contracts, net		0	0
Fuel derivatives | Other Assets
Offsetting Liabilities
Gross amounts of recognized liabilities		15	0
Gross amounts offset in the Balance Sheet		(15)	0
Liabilities derivative contracts, net	[1]	0	0
Interest rate derivatives | Accrued Liabilities
Offsetting Liabilities
Gross amounts of recognized liabilities		5	2
Gross amounts offset in the Balance Sheet		0	0
Liabilities derivative contracts, net		5	2
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities
Gross amounts of recognized liabilities		1	12
Gross amounts offset in the Balance Sheet		0	0
Liabilities derivative contracts, net		$ 1	$ 12

[1] The net amounts of derivative assets and liabilities are reconciled to the individual line item amounts presented in the Consolidated Balance Sheet in Note 15.