Fair Value Measurements - Measured on Recurring Basis (Details) - USD ($) $ in Millions		Sep. 30, 2018		Dec. 31, 2017
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset	[1]	$ 698		$ 283
Fuel derivatives:
Derivative Liability, Fair Value, Gross Liability	[1]	(22)		(57)
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Treasury bills		1,488		1,491
Fuel derivatives:
Other available for sale securities		141		107
Total assets		3,325		2,731
Liabilities
Interest rate derivatives (see Note 3)		0		0
Total Liabilities				0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Treasury bills		0		0
Fuel derivatives:
Other available for sale securities		0		0
Total assets		636		649
Liabilities
Interest rate derivatives (see Note 3)		(22)		(22)
Total Liabilities				(22)
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Treasury bills		0		0
Fuel derivatives:
Other available for sale securities		0		0
Total assets		698		283
Liabilities
Interest rate derivatives (see Note 3)		0		0
Total Liabilities				(35)
Options Held | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		0	[2]	0	[3]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability	[3]			0
Options Held | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		0	[2]	0	[3]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability	[3]			0
Options Held | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		698	[2]	283	[3]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability	[3]			(35)
Cash Equivalents | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		1,696	[4]	1,133	[5]
Cash Equivalents | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		0	[4]	0	[5]
Cash Equivalents | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0	[4]	0	[5]
Commercial Paper | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		0		0
Commercial Paper | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		369		350
Commercial Paper | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0		0
Certificates of Deposit | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		0		0
Investments		0		0
Certificates of Deposit | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		14		12
Investments		228		287
Certificates of Deposit | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0		0
Investments		0		0
Time Deposits | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		0
Time Deposits | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		25
Time Deposits | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring
Assets
Treasury bills		1,488		1,491
Fuel derivatives:
Other available for sale securities		141		107
Total assets		4,659		3,663
Liabilities
Interest rate derivatives (see Note 3)		(22)		(22)
Total Liabilities				(57)
Estimate of Fair Value Measurement | Options Held | Fair Value, Measurements, Recurring
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		698	[2]	283	[3]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability	[3]			(35)
Estimate of Fair Value Measurement | Cash Equivalents | Fair Value, Measurements, Recurring
Assets
Cash equivalents		1,696	[4]	1,133	[5]
Estimate of Fair Value Measurement | Commercial Paper | Fair Value, Measurements, Recurring
Assets
Cash equivalents		369		350
Estimate of Fair Value Measurement | Certificates of Deposit | Fair Value, Measurements, Recurring
Assets
Cash equivalents		14		12
Investments		228		$ 287
Estimate of Fair Value Measurement | Time Deposits | Fair Value, Measurements, Recurring
Assets
Cash equivalents		$ 25

[1] Represents the position of each trade before consideration of offsetting positions with each counterparty and does not include the impact of cash collateral deposits provided to or received from counterparties. See discussion of credit risk and collateral following in this Note.

[2] In the unaudited Condensed Consolidated Balance Sheet amounts are presented as an asset. See Note 3.

[3] In the unaudited Condensed Consolidated Balance Sheet amounts are presented as a net asset. See Note 3.

[4] Cash equivalents are primarily composed of money market investments.

[5] Cash equivalents are primarily composed of money market investments.