Derivatives and Risk Management - Hedge Position (Details) ¥ in Millions, gal in Millions, CAD in Millions, $ in Millions	Jun. 30, 2017 JPY (¥) gal	Jun. 30, 2017 CAD gal	Jun. 30, 2017 USD ($) gal	Dec. 31, 2016 JPY (¥) gal	Dec. 31, 2016 CAD gal	Dec. 31, 2016 USD ($) gal
Derivatives, Fair Value
Foreign currency exchange contracts, hedge derivatives, net			$ (7)			$ 27
Fuel hedge contracts, derivatives, net			(203)			(324)
Net derivative contracts, hedge derivatives, net			(214)			(291)
Prepaid Expenses and Other
Derivatives, Fair Value
Total derivative contracts, assets			163			393
Other Noncurrent Assets
Derivatives, Fair Value
Total derivative contracts, assets			1			7
Other Accrued Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities			(372)			(688)
Other Noncurrent Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities			(6)			(3)
Designated as hedging instrument
Derivatives, Fair Value
Interest rate contract (fair value hedge), hedge derivatives, net			(4)			6
Foreign currency exchange contracts, hedge derivatives, net			(7)			27
Not designated as hedging instrument
Derivatives, Fair Value
Fuel hedge contracts, derivatives, net			(203)			(324)
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			1			4
Interest rate contract (fair value hedge) | Designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Interest rate contract (fair value hedge), liabilities			(5)			0
Interest rate contract (fair value hedge) | Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Interest rate contract (fair value hedge), liabilities			0			0
Foreign currency exchange contracts | Designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets			11			31
Foreign currency exchange contracts | Designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets			0			3
Foreign currency exchange contracts | Designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			(12)			(4)
Foreign currency exchange contracts | Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			$ (6)			$ (3)
Fuel hedge contracts | Not designated as hedging instrument
Derivatives, Fair Value
Derivative, nonmonetary notional amount | gal	231	231	231	197	197	197
Fuel hedge contracts | Not designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Fuel hedge contracts, assets			$ 152			$ 360
Fuel hedge contracts | Not designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Fuel hedge contracts, assets			0			0
Fuel hedge contracts | Not designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities			(355)			(684)
Fuel hedge contracts | Not designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities			0			0
Fair value hedging | Interest rate contract (fair value hedge) | Designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount			$ 332			$ 349
Cash flow hedging | Foreign currency exchange contracts | Designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount	¥ 44,493	CAD 532		¥ 54,853	CAD 335