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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, 2017 USD ($) Claim | Dec. 31, 2016 USD ($) | [1] | Dec. 31, 2015 USD ($) | [1] | Dec. 31, 2014 USD ($) | [1] | Dec. 31, 2013 USD ($) | [1] | Dec. 31, 2012 USD ($) | [1] | Dec. 31, 2011 USD ($) | [1] | Dec. 31, 2010 USD ($) | [1] | Dec. 31, 2009 USD ($) | [1] | Dec. 31, 2008 USD ($) | [1]
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 2,119,907 |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | 98,504 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | 1,500,397 |  |  |  |  |  |  |  |  |
All outstanding claim liabilities before 2008, net of reinsurance |  | 67,446 |  |  |  |  |  |  |  |  |
Liabilities for claims and claim adjustment expenses, net of reinsurance |  | 686,956 |  |  |  |  |  |  |  |  |
Accident Year 2008 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | 182,646 | $ 182,901 | $ 183,608 | $ 182,846 | $ 184,547 | $ 187,695 | $ 190,905 | $ 195,619 | $ 200,371 | $ 196,923
Total IBNR Plus Expected Development on Reported Claims |  | $ 2,649 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 6,487 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 173,138 | 171,496 | 170,347 | 166,814 | 163,933 | 157,388 | 147,377 | 132,515 | 103,057 | $ 47,685
Claim Frequency | [2] | 20.98% |  |  |  |  |  |  |  |  |
Accident Year 2009 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 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 |  | $ 3,345 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,371 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 160,580 | 159,345 | 158,214 | 155,365 | 150,925 | 142,667 | 123,946 | 96,173 | $ 42,332 |
Claim Frequency | [2] | 19.82% |  |  |  |  |  |  |  |  |
Accident Year 2010 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 189,403 | 191,000 | 193,029 | 198,861 | 205,769 | 208,035 | 202,479 | 179,156 |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 5,805 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,971 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 175,019 | 172,426 | 169,565 | 165,834 | 156,974 | 141,029 | 108,714 | $ 47,520 |  |
Claim Frequency | [2] | 24.94% |  |  |  |  |  |  |  |  |
Accident Year 2011 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 191,126 | 192,988 | 195,262 | 198,213 | 199,163 | 199,522 | 196,384 |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 6,968 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 6,041 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 167,757 | 165,967 | 161,639 | 153,968 | 140,831 | 111,029 | $ 53,329 |  |  |
Claim Frequency | [2] | 22.81% |  |  |  |  |  |  |  |  |
Accident Year 2012 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 182,859 | 184,460 | 193,515 | 212,738 | 222,075 | 222,549 |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 7,840 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,746 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 157,206 | 154,553 | 149,161 | 133,658 | 107,467 | $ 50,579 |  |  |  |
Claim Frequency | [2] | 18.72% |  |  |  |  |  |  |  |  |
Accident Year 2013 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 214,701 | 220,457 | 233,656 | 241,811 | 241,810 |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 10,501 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,762 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 172,479 | 165,994 | 150,304 | 119,507 | $ 51,396 |  |  |  |  |
Claim Frequency | [2] | 16.54% |  |  |  |  |  |  |  |  |
Accident Year 2014 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 235,058 | 249,097 | 268,846 | 268,846 |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 13,503 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,827 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 169,736 | 153,320 | 119,820 | $ 53,060 |  |  |  |  |  |
Claim Frequency | [2] | 14.96% |  |  |  |  |  |  |  |  |
Accident Year 2015 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 252,514 | 262,573 | 262,573 |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 22,863 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,506 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 151,818 | 121,599 | $ 54,141 |  |  |  |  |  |  |
Claim Frequency | [2] | 14.22% |  |  |  |  |  |  |  |  |
Accident Year 2016 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 250,491 | 250,491 |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 17,066 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 5,346 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 115,713 | $ 52,238 |  |  |  |  |  |  |  |
Claim Frequency | [2] | 14.11% |  |  |  |  |  |  |  |  |
Accident Year 2017 [Member] |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 244,094 |  |  |  |  |  |  |  |  |
Total IBNR Plus Expected Development on Reported Claims |  | $ 7,964 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims Reported | Claim |  | 4,919 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  | $ 56,951 |  |  |  |  |  |  |  |  |
Claim Frequency | [2] | 13.86% |  |  |  |  |  |  |  |  |
[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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