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Financial Instruments and Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | $ 2,884 | $ 1,482
Restricted cash | 107 | 109
Equity securities | 147 | 99
Enhanced equipment trust certificates (EETC) | 19 | 22
Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 958 | 958
Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 722 | 753
U.S. government and agency notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 102 | 113
CDARS |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 49 | 120
Other fixed-income securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 170 | 188
Other Investments Measured At NAV |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 186 | 184
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 2,884 | 1,482
Restricted cash | 107 | 109
Equity securities | 147 | 99
Enhanced equipment trust certificates (EETC) | 0 | 0
Level 1 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 1 | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 1 | U.S. government and agency notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 1 | CDARS |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 1 | Other fixed-income securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 1 | Other Investments Measured At NAV |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 0 | 0
Restricted cash | 0 | 0
Equity securities | 0 | 0
Enhanced equipment trust certificates (EETC) | 0 | 0
Level 2 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 958 | 958
Level 2 | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 722 | 753
Level 2 | U.S. government and agency notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 102 | 113
Level 2 | CDARS |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 49 | 120
Level 2 | Other fixed-income securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 170 | 188
Level 2 | Other Investments Measured At NAV |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 0 | 0
Restricted cash | 0 | 0
Equity securities | 0 | 0
Enhanced equipment trust certificates (EETC) | 19 | 22
Level 3 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 3 | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 3 | U.S. government and agency notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 3 | CDARS |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 3 | Other fixed-income securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | 0 | 0
Level 3 | Other Investments Measured At NAV |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Short-term investments | $ 0 | $ 0

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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