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Financial Instruments and Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 6 Months Ended
--- | --- | ---
Apr. 30, 2018 | Jun. 30, 2018 | Jun. 30, 2017
Fair Value [Line Items] |  |  |
Amount invested in affiliate |  | $ 139 | $ 0
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) |  | 13 years |
Azul |  |  |
Fair Value [Line Items] |  |  |
Amount invested in affiliate | $ 138 |  |
Equity stake percentage | 2.00% |  |
Azul | Preferred Stock |  |  |
Fair Value [Line Items] |  |  |
Equity stake percentage | 8.00% |  |

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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
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