Fair Value Measurements - Schedule of Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure
Cash equivalents	$ 1,424	$ 2,279
Restricted cash equivalents and investments	60	61
Fuel hedge contracts	(238)	(324)
Interest rate contract	(5)	6
Foreign currency exchange contracts	1	27
U.S. government and agency securities
Assets, Fair Value Disclosure
Investments	95	112
Asset- and mortgage-backed securities
Assets, Fair Value Disclosure
Investments	159	68
Corporate obligations
Assets, Fair Value Disclosure
Investments	418	295
Other fixed income securities
Assets, Fair Value Disclosure
Investments	71	12
Long-Term Investments
Assets, Fair Value Disclosure
Investments	142	139
Level 1
Assets, Fair Value Disclosure
Cash equivalents	1,424	2,279
Restricted cash equivalents and investments	60	61
Fuel hedge contracts	(18)	(26)
Level 1 \| U.S. government and agency securities
Assets, Fair Value Disclosure
Investments	85	86
Level 1 \| Long-Term Investments
Assets, Fair Value Disclosure
Investments	117	115
Level 2
Assets, Fair Value Disclosure
Fuel hedge contracts	(220)	(298)
Interest rate contract	(5)	6
Foreign currency exchange contracts	1	27
Level 2 \| U.S. government and agency securities
Assets, Fair Value Disclosure
Investments	10	26
Level 2 \| Asset- and mortgage-backed securities
Assets, Fair Value Disclosure
Investments	159	68
Level 2 \| Corporate obligations
Assets, Fair Value Disclosure
Investments	418	295
Level 2 \| Other fixed income securities
Assets, Fair Value Disclosure
Investments	71	12
Level 2 \| Long-Term Investments
Assets, Fair Value Disclosure
Investments	$ 25	$ 24

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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)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624163-113959

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6526789

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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
-Glossary Available-for-Sale Securities
-URI http://asc.fasb.org/extlink&oid=6505594

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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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