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Investments - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) Security | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Schedule of Investments [Line Items] |  |  |
Identification of potential other-than-temporary impairments | 25.00% |  |
Impairment of investments | $ | $ 0.2 | $ 0.8 | $ 0.2
Number of marketable equity securities in an unrealized loss position | Security | 274 |  |
Investment [Member] | Issuer Concentration [Member] | Market Value of Total Investment Portfolio [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Investment concentration risk percentage | 5.00% |  |

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