Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		$ (1,028)	$ (1,028)	$ (1,218)	$ (1,059)
Total gains or (losses) (realized or unrealized)
Included in earnings		(85)	9
Included in other comprehensive income		50	(29)
Purchases		182	244
Sales		(127)	(175)
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2015		85	(8)
Fuel derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		(1,055)	(1,055)	$ (1,245)	$ (1,091)
Total gains or (losses) (realized or unrealized)
Included in earnings		(85)	8
Included in other comprehensive income		50	(29)
Purchases	[1]	182	244
Sales	[1]	(127)	(171)
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2015		85	(8)
Auction rate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		27	27
Total gains or (losses) (realized or unrealized)
Included in earnings		0	0
Included in other comprehensive income		0	0
Purchases		0	0
Sales		0	0
June 30, 2015	[2]	27	27
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2015		0	0
Other securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0	5
Total gains or (losses) (realized or unrealized)
Included in earnings		0	1
Included in other comprehensive income		0	0
Purchases		0	0
Sales		0	(4)
June 30, 2015		0	0
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2015		$ 0	$ 0

[1] The purchase and sale of fuel derivatives are recorded gross based on the structure of the derivative instrument and whether a contract with multiple derivatives is purchased as a single instrument or separate instruments.

[2] Included in Other assets in the unaudited Condensed Consolidated Balance Sheet.