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Fair Value Measurements and Investments - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017
EETC |
Fair Value [Line Items] |
Available-for-sale securities remaining maturities | 2019
Asset-backed Securities | Minimum |
Fair Value [Line Items] |
Available-for-sale securities remaining maturities | 1 year
Asset-backed Securities | Maximum |
Fair Value [Line Items] |
Available-for-sale securities remaining maturities | 17 years
Corporate Debt | Minimum |
Fair Value [Line Items] |
Available-for-sale securities remaining maturities | 1 year
Corporate Debt | Maximum |
Fair Value [Line Items] |
Available-for-sale securities remaining maturities | 3 years
CDARS | Maximum |
Fair Value [Line Items] |
Available-for-sale securities remaining maturities | 1 year
U.S. Government and Other Securities | Minimum |
Fair Value [Line Items] |
Available-for-sale securities remaining maturities | 1 year
U.S. Government and Other Securities | Maximum |
Fair Value [Line Items] |
Available-for-sale securities remaining maturities | 3 years

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