Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Jun. 30, 2017	Dec. 31, 2016
Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Liability derivative contracts, net		$ 24	$ 35
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		1	48
Gross asset amounts offset in the Balance Sheet		(1)	(48)
Liability derivative contracts, net		0	0
Fuel derivatives | Other Assets
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		34	58
Gross asset amounts offset in the Balance Sheet		(34)	(58)
Liability derivative contracts, net	[1]	0	0
Fuel derivatives | Accrued Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		515	674
Gross asset amounts offset in the Balance Sheet		(346)	(516)
Liability derivative contracts, net	[1]	169	158
Interest rate derivatives | Other Noncurrent Liabilities
Offsetting Liabilities [Line Items]
Gross amounts of recognized liabilities		24	35
Gross asset amounts offset in the Balance Sheet		0	0
Liability derivative contracts, net	[1]	$ 24	$ 35

[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 5.