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Financial Instruments and Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 9 Months Ended
--- | --- | ---
Apr. 30, 2018 | Sep. 30, 2018 | Sep. 30, 2017
Fair Value [Line Items] |  |  |
Amount invested |  | $ 1,975 | $ 2,584
Azul |  |  |
Fair Value [Line Items] |  |  |
Amount invested | $ 138 |  |
Equity stake percentage | 2.00% |  |
Azul | Preferred Stock |  |  |
Fair Value [Line Items] |  |  |
Equity stake percentage | 8.00% |  |
Minimum | Asset-backed Securities |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities (less than one year) |  | 1 year |
Minimum | Corporate Debt |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities (less than one year) |  | 1 year |
Minimum | U.S. Government and Other Securities |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities (less than one year) |  | 1 year |
Maximum | Asset-backed Securities |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities (less than one year) |  | 16 years |
Maximum | Corporate Debt |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities (less than one year) |  | 3 years |
Maximum | CDARS |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities (less than one year) |  | 1 year |
Maximum | U.S. Government and Other Securities |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities (less than one year) |  | 2 years |

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- Definition The cash outflow associated with the purchase of all investments (debt, security, other) during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 + Details
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