Fair Value Measurements - Schedule of Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Assets, Fair Value Disclosure
Cash equivalents	$ 1,080	$ 1,543
Restricted cash equivalents and investments	69	49
Fuel hedge contracts	(271)	(672)
Interest rate contract	8	(3)
Foreign currency exchange contracts	(64)	94
U.S. government and agency securities
Assets, Fair Value Disclosure
Investments	250	151
Asset- and mortgage-backed securities
Assets, Fair Value Disclosure
Investments	265	380
Corporate obligations
Assets, Fair Value Disclosure
Investments	880	896
Other fixed income securities
Assets, Fair Value Disclosure
Investments	119	38
Long-Term Investments
Assets, Fair Value Disclosure
Investments	161	155
Level 1
Assets, Fair Value Disclosure
Cash equivalents	1,080	1,543
Restricted cash equivalents and investments	69	49
Fuel hedge contracts	(4)	65
Level 1 \| U.S. government and agency securities
Assets, Fair Value Disclosure
Investments	170	74
Level 1 \| Long-Term Investments
Assets, Fair Value Disclosure
Investments	135	130
Level 2
Assets, Fair Value Disclosure
Fuel hedge contracts	(267)	(737)
Interest rate contract	8	(3)
Foreign currency exchange contracts	(64)	94
Level 2 \| U.S. government and agency securities
Assets, Fair Value Disclosure
Investments	80	77
Level 2 \| Asset- and mortgage-backed securities
Assets, Fair Value Disclosure
Investments	265	380
Level 2 \| Corporate obligations
Assets, Fair Value Disclosure
Investments	880	896
Level 2 \| Other fixed income securities
Assets, Fair Value Disclosure
Investments	119	38
Level 2 \| Long-Term Investments
Assets, Fair Value Disclosure
Investments	$ 26	$ 25

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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Fair value as of the balance sheet date of all foreign currency derivatives designated as cash flow hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(b),(c)(1)
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Fair value as of the balance sheet date of interest rate derivative assets, net of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized on the balance sheet.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=6526789

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Fair value portion of other assets.

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Fair value as of the balance sheet date of price risk derivative assets, net of price risk derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized on the balance sheet.

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