Derivatives and Risk Management - Hedge Position (Details) € in Millions, ₩ in Millions, gal in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020 KRW (₩) gal	Dec. 31, 2019 KRW (₩) gal	Mar. 31, 2020 EUR (€)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 EUR (€)	Dec. 31, 2019 USD ($)
Derivatives, Fair Value
Total derivative contracts, net				$ 47		$ 68
Prepaid Expenses and Other
Derivatives, Fair Value
Total derivative contracts, assets				112		38
Other Noncurrent Assets
Derivatives, Fair Value
Total derivative contracts, assets				25		53
Other Accrued Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities				(90)		(19)
Other Noncurrent Liabilities
Derivatives, Fair Value
Total derivative contracts, liabilities				0		(4)
Foreign exchange contracts - Euro based | Not designated as hedges
Derivatives, Fair Value
Volume | €			€ 238		€ 397
Foreign currency exchange contracts, net				11		9
Foreign exchange contracts - Euro based | Not designated as hedges | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets				11		9
Foreign exchange contracts - Euro based | Not designated as hedges | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets				0		0
Foreign exchange contracts - Euro based | Not designated as hedges | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities				0		0
Foreign exchange contracts - Euro based | Not designated as hedges | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities				0		0
Foreign exchange contracts - Won based | Not designated as hedges
Derivatives, Fair Value
Volume | ₩	₩ 177,045	₩ 177,045
Foreign currency exchange contracts, net				3		(3)
Foreign exchange contracts - Won based | Not designated as hedges | Prepaid Expenses and Other
Derivatives, Fair Value
Foreign currency exchange contracts, assets				1		1
Foreign exchange contracts - Won based | Not designated as hedges | Other Noncurrent Assets
Derivatives, Fair Value
Foreign currency exchange contracts, assets				2		0
Foreign exchange contracts - Won based | Not designated as hedges | Other Accrued Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities				0		0
Foreign exchange contracts - Won based | Not designated as hedges | Other Noncurrent Liabilities
Derivatives, Fair Value
Foreign currency exchange contracts, liabilities				0		(4)
Fuel hedge contracts | Not designated as hedges
Derivatives, Fair Value
Volume, nonmonetary | gal	219	243
Fuel hedge contracts, net				8		1
Fuel hedge contracts | Not designated as hedges | Prepaid Expenses and Other
Derivatives, Fair Value
Fuel hedge contracts, assets				98		16
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				(90)		(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
Volume				150		1,872
Interest rate contract, net				25		61
Fair value hedges | Interest rate contracts | Designated as hedges | Prepaid Expenses and Other
Derivatives, Fair Value
Interest rate contract, assets				2		12
Fair value hedges | Interest rate contracts | Designated as hedges | Other Noncurrent Assets
Derivatives, Fair Value
Interest rate contract, assets				23		53
Fair value hedges | Interest rate contracts | Designated as hedges | Other Accrued Liabilities
Derivatives, Fair Value
Interest rate contract, liabilities				0		(4)
Fair value hedges | Interest rate contracts | Designated as hedges | Other Noncurrent Liabilities
Derivatives, Fair Value
Interest rate contract, liabilities				$ 0		$ 0