Derivatives and Risk Management - Hedge Position (Details) € in Millions, ¥ in Millions, gal in Millions, $ in Millions, $ in Millions	Jun. 30, 2018 EUR (€) gal	Jun. 30, 2018 JPY (¥) gal	Jun. 30, 2018 USD ($) gal	Apr. 30, 2018 USD ($)	Jan. 31, 2018 EUR (€)	Dec. 31, 2017 JPY (¥) gal	Dec. 31, 2017 CAD ($) gal	Dec. 31, 2017 USD ($) gal
Derivatives, Fair Value
Total derivative contracts, net			$ (51)					$ (83)
Prepaid Expenses and Other
Derivatives, Fair Value
Total derivative contracts, assets			84					639
Other Noncurrent Assets
Derivatives, Fair Value
Total derivative contracts, assets			12					9
Other Accrued Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities			(106)					(707)
Other Noncurrent Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities			(41)					(24)
Interest rate contracts | Designated as hedging instrument
Derivatives, Fair Value
Interest rate contract, net			(24)
Interest rate contracts | Designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Interest rate contract, assets			4
Interest rate contracts | Designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Interest rate contract, assets			0
Interest rate contracts | Designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Interest rate contract, liabilities			(7)
Interest rate contracts | Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Interest rate contract, liabilities			(21)
Foreign currency exchange contracts | Designated as hedging instrument
Derivatives, Fair Value
Foreign currency exchange contracts, net			(1)					(17)
Foreign currency exchange contracts | Designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets			0					1
Foreign currency exchange contracts | Designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets			0					1
Foreign currency exchange contracts | Designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			(1)					(13)
Foreign currency exchange contracts | Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			0					$ (6)
Foreign currency exchange contracts | Not designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount | €	€ 375
Foreign currency exchange contracts, net			6
Foreign currency exchange contracts | Not designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets			10
Foreign currency exchange contracts | Not designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets			0
Foreign currency exchange contracts | Not designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			0
Foreign currency exchange contracts | Not designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			$ (4)
Fuel hedge contracts | Not designated as hedging instrument
Derivatives, Fair Value
Derivative, nonmonetary notional amount | gal	334	334	334			249	249	249
Fuel hedge contracts, net			$ (32)					$ (66)
Fuel hedge contracts | Not designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Fuel hedge contracts, assets			70					638
Fuel hedge contracts | Not designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Fuel hedge contracts, assets			12					8
Fuel hedge contracts | Not designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities			(98)					(694)
Fuel hedge contracts | Not designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities			(16)					$ (18)
Fair value hedge | Interest rate contracts | Designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount			$ 1,908	$ 1,600
Cash flow hedge | Foreign currency exchange contracts | Designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount		¥ 15,285				¥ 23,512	$ 490
Cash flow hedge | Foreign currency exchange contracts | Not designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount | €					€ 375