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Fair Value Measurements - Schedule of Fair Value Measurements on a Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Net Asset (Liability)
Cash equivalents	$ 1,508	$ 1,222
Restricted cash equivalents	1,075	1,183
Long-term investments	1,174	1,084
U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments		50
Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments		36
Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments		90
Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments		27
Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	7	15
Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	24	1
Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	8	(3)
Level 1
Fair Value, Net Asset (Liability)
Cash equivalents	1,508	1,222
Restricted cash equivalents	1,075	1,183
Long-term investments	970	880
Level 1 | U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments		45
Level 1 | Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments		0
Level 1 | Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments		0
Level 1 | Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments		0
Level 1 | Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	1	20
Level 1 | Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0	0
Level 1 | Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0	0
Level 2
Fair Value, Net Asset (Liability)
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	204	204
Level 2 | U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments		5
Level 2 | Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments		36
Level 2 | Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments		90
Level 2 | Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments		27
Level 2 | Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	6	(5)
Level 2 | Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	24	1
Level 2 | Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	$ 8	$ (3)

X

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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-Publisher FASB
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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 45
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The aggregate value of all restricted investments.

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-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
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