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Loss and Loss Adjustment Expenses - Development Tables of Incurred and Paid Claims and Allocated Claim Adjustment Expenses Net of Reinsurance (Detail) $ in Thousands | 12 Months Ended |  |  |  |  |  |  |  |  |
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Dec. 31, 2018 USD ($) Claim | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Dec. 31, 2009 USD ($)
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 2,142,468 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | 113,526 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | 1,525,214 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
All outstanding claim liabilities before 2008, net of reinsurance |  | 73,939 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities for claims and claim adjustment expenses, net of reinsurance |  | 691,193 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2009 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | 174,731 | $ 177,015 |  | $ 176,586 |  | $ 177,795 |  | $ 182,274 |  | $ 184,006 |  | $ 187,943 |  | $ 188,462 |  | $ 188,746 |  | $ 185,359 |
Total IBNR Plus Expected Development on Reported Claims |  | $ 3,407 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,372 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 162,253 | 160,580 | [1] | 159,345 | [1] | 158,214 | [1] | 155,365 | [1] | 150,925 | [1] | 142,667 | [1] | 123,946 | [1] | 96,173 | [1] | $ 42,332 | [1]
Claim Frequency | [2] | 19.82% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2010 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 189,040 | 189,403 |  | 191,000 |  | 193,029 |  | 198,861 |  | 205,769 |  | 208,035 |  | 202,479 |  | 179,156 |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 5,612 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,970 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 176,663 | 175,019 | [1] | 172,426 | [1] | 169,565 | [1] | 165,834 | [1] | 156,974 | [1] | 141,029 | [1] | 108,714 | [1] | $ 47,520 | [1] |  |
Claim Frequency | [2] | 24.93% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2011 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 189,327 | 191,126 |  | 192,988 |  | 195,262 |  | 198,213 |  | 199,163 |  | 199,522 |  | 196,384 |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 7,045 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 6,043 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 169,994 | 167,757 | [1] | 165,967 | [1] | 161,639 | [1] | 153,968 | [1] | 140,831 | [1] | 111,029 | [1] | $ 53,329 | [1] |  |  |  |
Claim Frequency | [2] | 22.81% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2012 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 180,387 | 182,859 |  | 184,460 |  | 193,515 |  | 212,738 |  | 222,075 |  | 222,549 |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 7,454 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,749 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 159,807 | 157,207 | [1] | 154,553 | [1] | 149,161 | [1] | 133,658 | [1] | 107,467 | [1] | $ 50,579 | [1] |  |  |  |  |  |
Claim Frequency | [2] | 18.73% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2013 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 210,588 | 214,701 |  | 220,457 |  | 233,656 |  | 241,811 |  | 241,810 |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 10,210 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,763 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 177,724 | 172,479 | [1] | 165,994 | [1] | 150,304 | [1] | 119,507 | [1] | $ 51,396 | [1] |  |  |  |  |  |  |  |
Claim Frequency | [2] | 16.54% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2014 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 226,933 | 235,058 |  | 249,097 |  | 268,846 |  | 268,846 |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 12,576 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,833 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 180,683 | 169,736 | [1] | 153,320 | [1] | 119,820 | [1] | $ 53,060 | [1] |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.97% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2015 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 235,471 | 252,514 |  | 262,573 |  | 262,573 |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 22,038 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,510 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 170,461 | 151,818 | [1] | 121,599 | [1] | $ 54,141 | [1] |  |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.23% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2016 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 241,406 | 250,491 |  | 250,491 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 19,874 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,378 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 143,016 | 115,713 | [1] | $ 52,238 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.19% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2017 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 244,098 | 244,094 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 9,887 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,178 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 122,552 | $ 56,951 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.58% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accident Year 2018 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | [1] | $ 250,487 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 15,423 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,186 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 62,061 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.42% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[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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