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Loss and Loss Adjustment Expenses - Development Tables of Incurred and Paid Losses and Allocated Loss Adjustment Expenses Net of Reinsurance (Detail) $ in Thousands | 12 Months Ended |  |  |  |  |  |  |  |  |
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Dec. 31, 2020 USD ($) Claim | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($)
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 2,124,295 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | 87,630 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | 1,549,975 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2011, net of reinsurance |  | 80,533 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities for loss and loss adjustment expenses, net of reinsurance |  | 654,854 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2011 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | 188,264 | $ 188,226 | [1] | $ 189,327 | [1] | $ 191,126 | [1] | $ 192,988 | [1] | $ 195,262 | [1] | $ 198,213 | [1] | $ 199,163 | [1] | $ 199,522 | [1] | $ 196,384 | [1]
Total IBNR Plus Expected Development on Reported Claims |  | $ 5,198 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 6,044 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 171,583 | 170,785 |  | 169,994 |  | 167,757 |  | 165,967 |  | 161,639 |  | 153,968 |  | 140,831 |  | 111,029 |  | $ 53,329 |
Claim Frequency | [2] | 22.82% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2012 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 177,547 | 178,586 | [1] | 180,387 | [1] | 182,859 | [1] | 184,460 | [1] | 193,515 | [1] | 212,738 | [1] | 222,075 | [1] | 222,549 | [1] |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 6,419 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,749 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 162,588 | 161,014 |  | 159,807 |  | 157,207 |  | 154,553 |  | 149,161 |  | 133,658 |  | 107,467 |  | $ 50,579 |  |  |
Claim Frequency | [2] | 18.73% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2013 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 207,304 | 209,184 | [1] | 210,588 | [1] | 214,701 | [1] | 220,457 | [1] | 233,656 | [1] | 241,811 | [1] | 241,810 | [1] |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 7,762 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,766 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 182,465 | 180,614 |  | 177,724 |  | 172,479 |  | 165,994 |  | 150,304 |  | 119,507 |  | $ 51,396 |  |  |  |  |
Claim Frequency | [2] | 16.55% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2014 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 212,417 | 218,386 | [1] | 226,933 | [1] | 235,058 | [1] | 249,097 | [1] | 268,846 | [1] | 268,846 | [1] |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 5,468 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,839 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 191,394 | 186,129 |  | 180,683 |  | 169,736 |  | 153,320 |  | 119,820 |  | $ 53,060 |  |  |  |  |  |  |
Claim Frequency | [2] | 14.99% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2015 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 211,758 | 220,965 | [1] | 235,471 | [1] | 252,514 | [1] | 262,573 | [1] | 262,573 | [1] |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 6,717 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,516 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 185,657 | 182,053 |  | 170,461 |  | 151,818 |  | 121,599 |  | $ 54,141 |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2016 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 209,214 | 218,005 | [1] | 241,406 | [1] | 250,491 | [1] | 250,491 | [1] |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 6,843 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,393 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 166,887 | 156,861 |  | 143,016 |  | 115,713 |  | $ 52,238 |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.23% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2017 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 220,096 | 234,587 | [1] | 244,098 | [1] | 244,094 | [1] |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 8,066 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,211 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 166,448 | 151,427 |  | 122,552 |  | $ 56,951 |  |  |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.68% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2018 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 235,641 | 250,487 | [1] | 250,487 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 16,057 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,467 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 152,328 | 126,057 |  | $ 62,061 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 15.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2019 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 241,344 | 241,344 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 16,609 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,199 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 120,512 | $ 58,883 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 15.21% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance |  | $ 220,710 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 8,491 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 4,170 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | [1] | $ 50,113 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 13.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Data presented for these calendar years is required supplementary information, which is unaudited
[2] | Frequency, as calculated above, refers to reported claims divided by gross premium earned

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