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Available-for-Sale Investments - Additional Information (Detail) $ in Millions | Dec. 31, 2018 USD ($) Investment Security | Dec. 31, 2017 USD ($)
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Investments Debt And Equity Securities [Abstract] |  |
Number of investments in unrealized loss position | 37 |
Number of investments in unrealized loss position, more than twelve months | 0 |
Aggregate fair value of securities in unrealized loss position | $ | $ 96.5 | $ 19.3
Number of securities with other than temporary impairment | Security | 0 |

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