Fair Value Measurements - Schedule of Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Net Asset (Liability)
Short-term investments	$ 478
Recurring
Fair Value, Net Asset (Liability)
Cash equivalents	982	$ 1,357
Recurring \| U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments	50	93
Recurring \| Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments	91	173
Recurring \| Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments	258	467
Recurring \| Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments	79	92
Recurring \| Restricted cash equivalents
Fair Value, Net Asset (Liability)
Restricted cash equivalents	1,268	38
Recurring \| Long-term investments
Fair Value, Net Asset (Liability)
Long-term investments	809	513
Recurring \| Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(29)	(66)
Recurring \| Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(38)
Recurring \| Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(9)	(17)
Recurring \| Level 1
Fair Value, Net Asset (Liability)
Cash equivalents	982	1,357
Recurring \| Level 1 \| U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments	45	84
Recurring \| Level 1 \| Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments	0	0
Recurring \| Level 1 \| Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments	0	0
Recurring \| Level 1 \| Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments	0	0
Recurring \| Level 1 \| Restricted cash equivalents
Fair Value, Net Asset (Liability)
Restricted cash equivalents	1,268	38
Recurring \| Level 1 \| Long-term investments
Fair Value, Net Asset (Liability)
Long-term investments	781	485
Recurring \| Level 1 \| Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(19)	(43)
Recurring \| Level 1 \| Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0
Recurring \| Level 1 \| Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0	0
Recurring \| Level 2
Fair Value, Net Asset (Liability)
Cash equivalents	0	0
Recurring \| Level 2 \| U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments	5	9
Recurring \| Level 2 \| Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments	91	173
Recurring \| Level 2 \| Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments	258	467
Recurring \| Level 2 \| Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments	79	92
Recurring \| Level 2 \| Restricted cash equivalents
Fair Value, Net Asset (Liability)
Restricted cash equivalents	0	0
Recurring \| Level 2 \| Long-term investments
Fair Value, Net Asset (Liability)
Long-term investments	28	28
Recurring \| Level 2 \| Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(10)	(23)
Recurring \| Level 2 \| Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(38)
Recurring \| Level 2 \| Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	$ (9)	$ (17)

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

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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-Publisher FASB
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-Topic 320
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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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The aggregate value of all restricted investments.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
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