Fair Value Measurements - Measured on Recurring Basis (Details) - USD ($)		Mar. 31, 2021	Dec. 31, 2020
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		$ 255,000,000	$ 134,000,000
Fuel derivatives:
Derivative Liability, Fair Value, Gross Liability		0	(6,000,000)
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Treasury bills		1,800,000,000	1,800,000,000
Interest rate derivatives (see Note 4)		0
Fuel derivatives:
Other available for sale securities		240,000,000	259,000,000
Total assets		13,640,000,000	12,722,000,000
Liabilities
Interest rate derivatives (see Note 4)			0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Treasury bills		0	0
Interest rate derivatives (see Note 4)		6,000,000
Fuel derivatives:
Other available for sale securities		0	0
Total assets		954,000,000	871,000,000
Liabilities
Interest rate derivatives (see Note 4)			(6,000,000)
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Treasury bills		0	0
Interest rate derivatives (see Note 4)		0
Fuel derivatives:
Other available for sale securities		0	0
Total assets		249,000,000	134,000,000
Liabilities
Interest rate derivatives (see Note 4)			0
Options Held | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Options Held | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Options Held | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset	[1]	249,000,000	134,000,000
Cash Equivalents | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents	[2]	11,600,000,000	10,663,000,000
Cash Equivalents | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents	[2]	0	0
Cash Equivalents | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents	[2]	0	0
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		90,000,000	90,000,000
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		6,000,000	10,000,000
Investments		27,000,000	46,000,000
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	0
Investments		0	0
Time Deposits | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		275,000,000	300,000,000
Investments		550,000,000	425,000,000
Time Deposits | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0	0
Investments		0	0
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring
Assets
Treasury bills		1,800,000,000	1,800,000,000
Interest rate derivatives (see Note 4)		6,000,000
Fuel derivatives:
Other available for sale securities		240,000,000	259,000,000
Total assets		14,843,000,000	13,727,000,000
Liabilities
Interest rate derivatives (see Note 4)			(6,000,000)
Estimate of Fair Value Measurement | Options Held | Fair Value, Measurements, Recurring
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset	[1]	249,000,000	134,000,000
Estimate of Fair Value Measurement | Cash Equivalents | Fair Value, Measurements, Recurring
Assets
Cash equivalents	[2]	11,600,000,000	10,663,000,000
Estimate of Fair Value Measurement | Commercial Paper | Fair Value, Measurements, Recurring
Assets
Cash equivalents		90,000,000	90,000,000
Estimate of Fair Value Measurement | Certificates of Deposit | Fair Value, Measurements, Recurring
Assets
Cash equivalents		6,000,000	10,000,000
Investments		27,000,000	46,000,000
Estimate of Fair Value Measurement | Time Deposits | Fair Value, Measurements, Recurring
Assets
Cash equivalents		275,000,000	300,000,000
Investments		$ 550,000,000	$ 425,000,000

[1] In the unaudited Condensed Consolidated Balance Sheet amounts are presented as an asset. See Note 4.

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