Fair Value Measurements - Measured on Recurring Basis (Details) - USD ($) $ in Millions		Sep. 30, 2017		Dec. 31, 2016
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset	[1]	$ 234		$ 444
Fuel derivatives:
Derivative Liability, Fair Value, Gross Liability	[1]	(265)		(805)
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Treasury bills		1,344		1,345
Fuel derivatives:
Other available for sale securities		101		83
Total assets		2,470		2,772
Liabilities
Interest rate derivatives (see Note 3)		0		0
Total Liabilities		0		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		683		658
Liabilities
Interest rate derivatives (see Note 3)		(21)		(35)
Total Liabilities		(48)		(145)
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		222		402
Liabilities
Interest rate derivatives (see Note 3)		0		0
Total Liabilities		(217)		(660)
Cash Equivalents | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		1,025	[2]	1,344	[3]
Cash Equivalents | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		0	[2]	0	[3]
Cash Equivalents | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0	[2]	0	[3]
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		420		325
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
Certificates of deposit | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		15		11
Certificates of deposit | 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
Investments		0		0
Certificates of deposit | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Investments		236		280
Certificates of deposit | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Investments		0		0
Swap | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fuel derivatives:
Derivative Liability, Fair Value, Gross Asset		0	[4]	0	[5]
Fuel derivatives:
Derivative Liability, Fair Value, Gross Liability		0	[4]	0	[5]
Swap | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fuel derivatives:
Derivative Liability, Fair Value, Gross Asset		12	[4]	42	[5]
Fuel derivatives:
Derivative Liability, Fair Value, Gross Liability		(27)	[4]	(110)	[5]
Swap | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fuel derivatives:
Derivative Liability, Fair Value, Gross Asset		0	[4]	0	[5]
Fuel derivatives:
Derivative Liability, Fair Value, Gross Liability		0	[4]	0	[5]
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	[6]	0	[7]
Derivative Liability, Fair Value, Gross Asset		0	[4]	0	[5]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		0	[6]	0	[7]
Derivative Liability, Fair Value, Gross Liability		0	[4]	0	[5]
Options Held | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		0	[6]	0	[7]
Derivative Liability, Fair Value, Gross Asset		0	[4]	0	[5]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		0	[6]	0	[7]
Derivative Liability, Fair Value, Gross Liability		0	[4]	0	[5]
Options Held | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		164	[6]	239	[7]
Derivative Liability, Fair Value, Gross Asset		58	[4]	163	[5]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		(76)	[6]	(96)	[7]
Derivative Liability, Fair Value, Gross Liability		(141)	[4]	(564)	[5]
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring
Assets
Treasury bills		1,344		1,345
Fuel derivatives:
Other available for sale securities		101		83
Total assets		3,375		3,832
Liabilities
Interest rate derivatives (see Note 3)		(21)		(35)
Total Liabilities		(265)		(805)
Estimate of Fair Value Measurement [Member] | Cash Equivalents | Fair Value, Measurements, Recurring
Assets
Cash equivalents		1,025	[2]	1,344	[3]
Estimate of Fair Value Measurement [Member] | Commercial paper | Fair Value, Measurements, Recurring
Assets
Cash equivalents		420		325
Estimate of Fair Value Measurement [Member] | Certificates of deposit | Fair Value, Measurements, Recurring
Assets
Cash equivalents		15		11
Estimate of Fair Value Measurement [Member] | Certificates of deposit | Fair Value, Measurements, Recurring
Assets
Investments		236		280
Estimate of Fair Value Measurement [Member] | Swap | Fair Value, Measurements, Recurring
Fuel derivatives:
Derivative Liability, Fair Value, Gross Asset		12	[4]	42	[5]
Fuel derivatives:
Derivative Liability, Fair Value, Gross Liability		(27)	[4]	(110)	[5]
Estimate of Fair Value Measurement [Member] | Options Held | Fair Value, Measurements, Recurring
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		164	[6]	239	[7]
Derivative Liability, Fair Value, Gross Asset		58	[4]	163	[5]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		(76)	[6]	(96)	[7]
Derivative Liability, Fair Value, Gross Liability		$ (141)	[4]	$ (564)	[5]

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

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

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

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

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

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