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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)

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- 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) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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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. + References No definition available. + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + Details
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- Definition 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 -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
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