Derivatives and Risk Management - Hedge Position (Details) ¥ in Millions, gal in Millions, CAD in Millions, $ in Millions	Sep. 30, 2017 JPY (¥) gal	Sep. 30, 2017 CAD gal	Sep. 30, 2017 USD ($) gal	Dec. 31, 2016 JPY (¥) gal	Dec. 31, 2016 CAD gal	Dec. 31, 2016 USD ($) gal
Derivatives, Fair Value
Total derivative contracts, net			$ (165)			$ (291)
Prepaid Expenses and Other
Derivatives, Fair Value
Total derivative contracts, assets			213			393
Other Noncurrent Assets
Derivatives, Fair Value
Total derivative contracts, assets			4			7
Other Accrued Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities			(352)			(688)
Other Noncurrent Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities			(30)			(3)
Interest rate contract (fair value hedge) | Designated as hedging instrument
Derivatives, Fair Value
Interest rate contract (fair value hedge), hedge derivatives, net			(4)			6
Interest rate contract (fair value hedge) | Designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Interest rate contract (fair value hedge), assets			0			2
Interest rate contract (fair value hedge) | Designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Interest rate contract (fair value hedge), assets			0			4
Interest rate contract (fair value hedge) | Designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Interest rate contract (fair value hedge), liabilities			0			0
Interest rate contract (fair value hedge) | Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Interest rate contract (fair value hedge), liabilities			(4)			0
Foreign currency exchange contracts | Designated as hedging instrument
Derivatives, Fair Value
Foreign currency exchange contracts, hedge derivatives, net			(22)			27
Foreign currency exchange contracts | Designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets			3			31
Foreign currency exchange contracts | Designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets			1			3
Foreign currency exchange contracts | Designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			(17)			(4)
Foreign currency exchange contracts | Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			$ (9)			$ (3)
Fuel hedge contracts | Not designated as hedging instrument
Derivatives, Fair Value
Derivative, nonmonetary notional amount | gal	288	288	288	197	197	197
Fuel hedge contracts, derivatives, net			$ (139)			$ (324)
Fuel hedge contracts | Not designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Fuel hedge contracts, assets			210			360
Fuel hedge contracts | Not designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Fuel hedge contracts, assets			3			0
Fuel hedge contracts | Not designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities			(335)			(684)
Fuel hedge contracts | Not designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities			(17)			0
Fair value hedging | Interest rate contract (fair value hedge) | Designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount			$ 320			$ 349
Cash flow hedging | Foreign currency exchange contracts | Designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount	¥ 29,396	CAD 563		¥ 54,853	CAD 335