Derivatives - Hedge Position (Details) ¥ in Millions, gal in Millions, CAD in Millions, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015 JPY (¥) gal	Dec. 31, 2014 JPY (¥) gal	Jun. 30, 2015 CAD gal	Jun. 30, 2015 USD ($) gal	Dec. 31, 2014 CAD gal	Dec. 31, 2014 USD ($) gal
Derivatives, Fair Value [Line Items]
Foreign currency exchange contracts, hedge derivatives, net				$ 89		$ 73
Fuel hedge contracts, derivatives, net				(758)		(1,848)
Net derivative contracts, hedge derivatives, net				(677)		(1,782)
Hedge Derivatives Asset
Derivatives, Fair Value [Line Items]
Total derivative contracts, assets				966		1,078
Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Total derivative contracts, assets				149		52
Hedge Derivatives Liability
Derivatives, Fair Value [Line Items]
Total derivative contracts, liabilities				(1,440)		(2,772)
Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Total derivative contracts, liabilities				(352)		(140)
Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Interest rate contract (fair value hedge), hedge derivatives, net				(8)		(7)
Foreign currency exchange contracts, hedge derivatives, net				89		73
Designated as hedging instrument | Hedge Derivatives Asset
Derivatives, Fair Value [Line Items]
Interest rate contract (fair value hedge), assets				5		5
Foreign currency exchange contracts, assets				55		25
Designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Interest rate contract (fair value hedge), assets				0		0
Foreign currency exchange contracts, assets				34		49
Designated as hedging instrument | Hedge Derivatives Liability
Derivatives, Fair Value [Line Items]
Interest rate contract (fair value hedge), liabilities				0		0
Foreign currency exchange contracts, liabilities				0		(1)
Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Interest rate contract (fair value hedge), liabilities				(13)		(12)
Foreign currency exchange contracts, liabilities				0		0
Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, derivatives, net				(758)		(1,848)
Not designated as hedging instrument | Hedge Derivatives Asset
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, assets				906		1,048
Not designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, assets				115		3
Not designated as hedging instrument | Hedge Derivatives Liability
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, liabilities				(1,440)		(2,771)
Not designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, liabilities				$ (339)		$ (128)
Fuel hedge contracts | Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Derivative, nonmonetary notional amount | gal	1,176	3,286	1,176	1,176	3,286	3,286
Derivative, final maturity date	Dec. 31, 2016	Dec. 31, 2016
Fair value hedging | Interest rate contract (fair value hedge) | Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount				$ 400		$ 416
Derivative, final maturity date	Aug. 31, 2022	Aug. 31, 2022
Cash flow hedging | Foreign currency exchange contracts | Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount	¥ 61,573	¥ 77,576	CAD 500		CAD 511
Derivative, final maturity date	Apr. 05, 2018	Oct. 05, 2017