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Investments - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2018 USD ($) Security | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Investment [Line Items] |  |  |
Short-term investments | $ 14,231,000 | $ 25,770,000 |
Sale of securities classified as held to maturity |  |  | $ 3,000,000
Gain of held-to-maturity investment securities |  |  | 100,000
Individual fixed maturity held in unrealized loss position | Security | 398 |  |
Individual fixed maturity held in unrealized loss position longer than 12 months | Security | 323 |  |
Impairment losses recognized | $ 0 | 0 | $ 0
Held-to-Maturity Securities [Member] |  |  |
Investment [Line Items] |  |  |
Investments on deposit with regulatory agencies | 16,100,000 |  |
Available-for-Sale Securities [Member] |  |  |
Investment [Line Items] |  |  |
Investments on deposit with regulatory agencies | 2,500,000 |  |
Corporate Bonds [Member] |  |  |
Investment [Line Items] |  |  |
Short-term investments | 13,400,000 |  |
States and Political Subdivisions [Member] |  |  |
Investment [Line Items] |  |  |
Short-term investments | $ 800,000 |  |
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |  |
Investment [Line Items] |  |  |
Short-term investments |  | $ 25,800,000 |

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