Derivatives and Risk Management - Hedge Position (Details) € in Millions, ¥ in Millions, gal in Millions, $ in Millions, $ in Millions	Mar. 31, 2019 EUR (€) gal	Mar. 31, 2019 JPY (¥) gal	Mar. 31, 2019 USD ($) gal	Dec. 31, 2018 EUR (€) gal	Dec. 31, 2018 CAD ($) gal	Dec. 31, 2018 USD ($) gal
Derivatives, Fair Value
Total derivative contracts, net			$ 39			$ 13
Prepaid Expenses and Other
Derivatives, Fair Value
Total derivative contracts, assets			63			44
Other Noncurrent Assets
Derivatives, Fair Value
Total derivative contracts, assets			28			8
Other Accrued Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities			(46)			(22)
Other Noncurrent Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities			(6)			(17)
Interest rate contracts | Designated as hedging instrument
Derivatives, Fair Value
Interest rate contract, net			24			1
Interest rate contracts | Designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Interest rate contract, assets			1			0
Interest rate contracts | Designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Interest rate contract, assets			28			8
Interest rate contracts | Designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Interest rate contract, liabilities			(2)			(7)
Interest rate contracts | Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Interest rate contract, liabilities			(3)			0
Foreign currency exchange contracts | Designated as hedging instrument
Derivatives, Fair Value
Foreign currency exchange contracts, net			1			1
Foreign currency exchange contracts | Designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets			1			1
Foreign currency exchange contracts | Designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets			0			0
Foreign currency exchange contracts | Designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			0			0
Foreign currency exchange contracts | Designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			0			0
Foreign currency exchange contracts | Not designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount | €	€ 397			€ 397
Foreign currency exchange contracts, net			7			(4)
Foreign currency exchange contracts | Not designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets			10			13
Foreign currency exchange contracts | Not designated as hedging instrument | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets			0			0
Foreign currency exchange contracts | Not designated as hedging instrument | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			0			0
Foreign currency exchange contracts | Not designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities			$ (3)			$ (17)
Fuel hedge contracts | Not designated as hedging instrument
Derivatives, Fair Value
Derivative, nonmonetary notional amount | gal	195	195	195	219	219	219
Fuel hedge contracts, net			$ 7			$ 15
Fuel hedge contracts | Not designated as hedging instrument | Prepaid Expenses and Other
Derivatives, Fair Value
Fuel hedge contracts, assets			51			30
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			(44)			(15)
Fuel hedge contracts | Not designated as hedging instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities			0			0
Fair value hedge | Interest rate contracts | Designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount			$ 1,883			$ 1,893
Cash flow hedge | Foreign currency exchange contracts | Designated as hedging instrument
Derivatives, Fair Value
Derivative, notional amount		¥ 4,328			$ 6,934