Derivatives and Risk Management - Hedge Position (Details) € in Millions, ¥ in Millions, gal in Millions, $ in Millions	Sep. 30, 2019 EUR (€) gal	Sep. 30, 2019 USD ($) gal	Dec. 31, 2018 EUR (€) gal	Dec. 31, 2018 JPY (¥) gal	Dec. 31, 2018 USD ($) gal
Derivatives, Fair Value
Total derivative contracts, net		$ 125			$ 13
Prepaid Expenses and Other
Derivatives, Fair Value
Total derivative contracts, assets		63			44
Other Noncurrent Assets
Derivatives, Fair Value
Total derivative contracts, assets		87			8
Other Accrued Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities		(25)			(22)
Other Noncurrent Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities		0			(17)
Foreign currency exchange contracts | Not designated as hedges
Derivatives, Fair Value
Derivative, notional amount | €	€ 397		€ 397
Foreign currency exchange contracts, net		21			(4)
Foreign currency exchange contracts | Not designated as hedges | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets		9			13
Foreign currency exchange contracts | Not designated as hedges | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets		12			0
Foreign currency exchange contracts | Not designated as hedges | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities		0			0
Foreign currency exchange contracts | Not designated as hedges | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities		$ 0			$ (17)
Fuel hedge contracts | Not designated as hedges
Derivatives, Fair Value
Derivative, nonmonetary notional amount | gal	14	14	219	219	219
Fuel hedge contracts, net		$ 22			$ 15
Fuel hedge contracts | Not designated as hedges | Prepaid Expenses and Other
Derivatives, Fair Value
Fuel hedge contracts, assets		47			30
Fuel hedge contracts | Not designated as hedges | Other Noncurrent Assets
Derivatives, Fair Value
Fuel hedge contracts, assets		0			0
Fuel hedge contracts | Not designated as hedges | Other Accrued Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities		(25)			(15)
Fuel hedge contracts | Not designated as hedges | Other Noncurrent Liabilities
Derivatives, Fair Value
Fuel hedge contracts, liabilities		0			0
Fair value hedges | Interest rate contracts | Designated as hedges
Derivatives, Fair Value
Derivative, notional amount		1,872			1,893
Interest rate contract, net		82			1
Fair value hedges | Interest rate contracts | Designated as hedges | Prepaid Expenses and Other
Derivatives, Fair Value
Interest rate contract, assets		7			0
Fair value hedges | Interest rate contracts | Designated as hedges | Other Noncurrent Assets
Derivatives, Fair Value
Interest rate contract, assets		75			8
Fair value hedges | Interest rate contracts | Designated as hedges | Other Accrued Liabilities
Derivatives, Fair Value
Interest rate contract, liabilities		0			(7)
Fair value hedges | Interest rate contracts | Designated as hedges | Other Noncurrent Liabilities
Derivatives, Fair Value
Interest rate contract, liabilities		$ 0			0
Cash flow hedges | Foreign currency exchange contracts | Designated as hedges
Derivatives, Fair Value
Derivative, notional amount | ¥				¥ 6,934
Foreign currency exchange contracts, net					1
Cash flow hedges | Foreign currency exchange contracts | Designated as hedges | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets					1
Cash flow hedges | Foreign currency exchange contracts | Designated as hedges | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets					0
Cash flow hedges | Foreign currency exchange contracts | Designated as hedges | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities					0
Cash flow hedges | Foreign currency exchange contracts | Designated as hedges | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities					$ 0