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Reinsurance - Schedule of the Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Premiums Written and Earned [Abstract] |  |  |  |  |  |  |  |  |  |  |
Gross Written |  |  |  |  |  |  |  |  | $ 303,090 | $ 333,460 | $ 351,696
Ceded Written |  |  |  |  |  |  |  |  | (10,276) | (8,995) | (9,344)
Net premiums Written |  |  |  |  |  |  |  |  | 292,814 | 324,465 | 342,352
Gross Earned |  |  |  |  |  |  |  |  | 314,703 | 341,883 | 359,670
Ceded Earned |  |  |  |  |  |  |  |  | (10,276) | (8,995) | (9,344)
Net premiums Earned | $ 74,702 | $ 74,771 | $ 75,964 | $ 78,990 | $ 82,277 | $ 82,712 | $ 82,951 | $ 84,948 | $ 304,427 | $ 332,888 | $ 350,326

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