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Derivatives and Risk Management - Hedge Position and Offsetting (Details) ¥ in Millions, gal in Millions, CAD in Millions, $ in Millions	Dec. 31, 2017 JPY (¥) gal	Dec. 31, 2017 CAD gal	Dec. 31, 2017 USD ($) gal	Dec. 31, 2016 JPY (¥) gal	Dec. 31, 2016 CAD gal	Dec. 31, 2016 USD ($) gal
Derivatives, Fair Value [Line Items]
Net derivative contracts, hedge derivatives, net			$ (83)			$ (297)
Hedge derivative asset
Derivatives, Fair Value [Line Items]
Total derivative contracts, assets			639			391
Net derivative contracts, assets			0			29
Other noncurrent assets
Derivatives, Fair Value [Line Items]
Total derivative contracts, assets			9			3
Net derivative contracts, assets			1			2
Hedge derivative liability
Derivatives, Fair Value [Line Items]
Total derivative contracts, liabilities			(707)			(688)
Net derivative contracts, liabilities			(68)			(326)
Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Total derivative contracts, liabilities			(24)			(3)
Net derivative contracts, liabilities			(16)			(2)
Foreign currency exchange contract \| Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Foreign currency exchange contracts			(17)			27
Foreign currency exchange contract \| Designated as hedging instrument \| Hedge derivative asset
Derivatives, Fair Value [Line Items]
Foreign currency exchange contracts, assets			1			31
Foreign currency exchange contract \| Designated as hedging instrument \| Other noncurrent assets
Derivatives, Fair Value [Line Items]
Foreign currency exchange contracts, assets			1			3
Foreign currency exchange contract \| Designated as hedging instrument \| Hedge derivative liability
Derivatives, Fair Value [Line Items]
Foreign currency exchange contracts, liabilities			(13)			(4)
Foreign currency exchange contract \| Designated as hedging instrument \| Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Foreign currency exchange contracts, liabilities			$ (6)			$ (3)
Foreign currency exchange contract \| Cash flow hedging \| Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount	¥ 23,512	CAD 490		¥ 54,853	CAD 335
Fuel hedge contracts \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Derivative, nonmonetary notional amount \| gal	249	249	249	197	197	197
Fuel hedge contracts, hedge derivatives, net			$ (66)			$ (324)
Fuel hedge contracts \| Not designated as hedging instrument \| Hedge derivative asset
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, assets			638			360
Fuel hedge contracts \| Not designated as hedging instrument \| Other noncurrent assets
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, assets			8			0
Fuel hedge contracts \| Not designated as hedging instrument \| Hedge derivative liability
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, liabilities			(694)			(684)
Fuel hedge contracts \| Not designated as hedging instrument \| Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Fuel hedge contracts, liabilities			$ (18)			$ 0