Derivatives - Hedge Position (Details) In Millions, unless otherwise specified																															3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2015 Hedge Derivatives Asset USD ($)	Dec. 31, 2014 Hedge Derivatives Asset USD ($)	Mar. 31, 2015 Other Noncurrent Assets USD ($)	Dec. 31, 2014 Other Noncurrent Assets USD ($)	Mar. 31, 2015 Hedge Derivatives Liability USD ($)	Dec. 31, 2014 Hedge Derivatives Liability USD ($)	Mar. 31, 2015 Other Noncurrent Liabilities USD ($)	Dec. 31, 2014 Other Noncurrent Liabilities USD ($)	Mar. 31, 2015 Designated as hedging instrument USD ($)	Dec. 31, 2014 Designated as hedging instrument USD ($)	Mar. 31, 2015 Designated as hedging instrument Hedge Derivatives Asset USD ($)	Dec. 31, 2014 Designated as hedging instrument Hedge Derivatives Asset USD ($)	Mar. 31, 2015 Designated as hedging instrument Other Noncurrent Assets USD ($)	Dec. 31, 2014 Designated as hedging instrument Other Noncurrent Assets USD ($)	Mar. 31, 2015 Designated as hedging instrument Hedge Derivatives Liability USD ($)	Dec. 31, 2014 Designated as hedging instrument Hedge Derivatives Liability USD ($)	Mar. 31, 2015 Designated as hedging instrument Other Noncurrent Liabilities USD ($)	Dec. 31, 2014 Designated as hedging instrument Other Noncurrent Liabilities USD ($)	Mar. 31, 2015 Not designated as hedging instrument USD ($)	Dec. 31, 2014 Not designated as hedging instrument USD ($)	Mar. 31, 2015 Not designated as hedging instrument Hedge Derivatives Asset USD ($)	Dec. 31, 2014 Not designated as hedging instrument Hedge Derivatives Asset USD ($)	Mar. 31, 2015 Not designated as hedging instrument Other Noncurrent Assets USD ($)	Dec. 31, 2014 Not designated as hedging instrument Other Noncurrent Assets USD ($)	Mar. 31, 2015 Not designated as hedging instrument Hedge Derivatives Liability USD ($)	Dec. 31, 2014 Not designated as hedging instrument Hedge Derivatives Liability USD ($)	Mar. 31, 2015 Not designated as hedging instrument Other Noncurrent Liabilities USD ($)	Dec. 31, 2014 Not designated as hedging instrument Other Noncurrent Liabilities USD ($)	Mar. 31, 2015 Fuel hedge contracts Not designated as hedging instrument gal	Dec. 31, 2014 Fuel hedge contracts Not designated as hedging instrument gal	Mar. 31, 2015 Fair value hedging Interest rate contract (fair value hedge) Designated as hedging instrument USD ($)	Dec. 31, 2014 Fair value hedging Interest rate contract (fair value hedge) Designated as hedging instrument USD ($)	Mar. 31, 2015 Cash flow hedging Foreign currency exchange contracts Designated as hedging instrument CAD	Mar. 31, 2015 Cash flow hedging Foreign currency exchange contracts Designated as hedging instrument JPY (¥)	Dec. 31, 2014 Cash flow hedging Foreign currency exchange contracts Designated as hedging instrument CAD	Dec. 31, 2014 Cash flow hedging Foreign currency exchange contracts Designated as hedging instrument JPY (¥)
Derivatives, Fair Value [Line Items]
Derivative, notional amount																																	$ 400	$ 416	506	¥ 63,914	511	¥ 77,576
Derivative, nonmonetary notional amount																															1,618,000,000	3,286,000,000
Derivative, final maturity date																															Dec. 31, 2016	Dec. 31, 2016	Aug. 31, 2022	Aug. 31, 2022	Jan. 05, 2018	Jan. 05, 2018	Oct. 05, 2017	Oct. 05, 2017
Interest rate contract (fair value hedge), assets													5	5	0	0
Interest rate contract (fair value hedge), liabilities																	0	0	(6)	(12)
Interest rate contract (fair value hedge), hedge derivatives, net											(1)	(7)
Foreign currency exchange contracts, assets													48	25	48	49
Foreign currency exchange contracts, liabilities																	0	(1)	0	0
Foreign currency exchange contracts, hedge derivatives, net	96	73									96	73
Fuel hedge contracts, assets	300																						1,522	1,048	35	3
Fuel hedge contracts, liabilities	(300)																										(2,512)	(2,771)	(473)	(128)
Fuel hedge contracts, derivatives, net	(1,428)	(1,848)																			(1,428)	(1,848)
Total derivative contracts, assets			1,575	1,078	83	52
Total derivative contracts, liabilities							(2,512)	(2,772)	(479)	(140)
Net derivative contracts, hedge derivatives, net	(1,333)	(1,782)
Net derivative contracts, assets			148	29	48	49
Net derivative contracts, liabilities							$ (1,074)	$ (1,723)	$ (455)	$ (137)