Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 234	$ 444
Derivative Liability, Fair Value, Gross Liability	[1]	265	805
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	124	137
Derivative Liability, Fair Value, Gross Liability	[1]	138	491
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	110	307
Derivative Liability, Fair Value, Gross Liability	[1]	127	314
Fuel derivatives
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		14
Fuel derivatives | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Total Collateral Already Received Aggregate Fair Value		3	4
Fuel derivatives | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		18	311
Fuel derivatives | Accounts Payable
Derivatives, Fair Value [Line Items]
Due To Third Parties For Settled Fuel Contracts		50	75
Fuel derivatives | Other Assets
Derivatives, Fair Value [Line Items]
Total Collateral Already Received Aggregate Fair Value		1	6
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	23	7
Derivative Asset, Fair Value, Gross Liability	[1]	36	44
Fuel derivatives | Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	16	4
Derivative Liability, Fair Value, Gross Liability	[1]	84	412
Fuel derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	85	126
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Fuel derivatives | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	41	54
Derivative Asset, Fair Value, Gross Liability	[1]	25	0
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	55	201
Derivative Liability, Fair Value, Gross Liability	[1]	85	262
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	14	52
Derivative Asset, Fair Value, Gross Liability	[1]	14	52
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	0	0
Derivative Liability, Fair Value, Gross Liability	[1]	18	35
Interest rate derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	0	0
Derivative Liability, Fair Value, Gross Liability	[1]	$ 3	$ 0

[1] Represents the position of each trade before consideration of offsetting positions with each counterparty and does not include the impact of cash collateral deposits provided to or received from counterparties. See discussion of credit risk and collateral following in this Note.