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Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Detail) $ in Thousands | Dec. 31, 2022 USD ($) Claim | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
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Commercial Specialty | Property Insurance |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 242,719 |  |  |  |  |  |  |  |  |
Commercial Specialty | Property Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | 77,860 | $ 78,760 | $ 75,609 |  |  |  |  |  |  |
IBNR | [1] | $ 2,003 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 4,598 |  |  |  |  |  |  |  |  |
Commercial Specialty | Property Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 82,841 | 84,037 |  |  |  |  |  |  |  |
IBNR | [1] | $ 5,712 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 4,206 |  |  |  |  |  |  |  |  |
Commercial Specialty | Property Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 82,018 |  |  |  |  |  |  |  |  |
IBNR | [1] | $ 14,533 |  |  |  |  |  |  |  |  |
Commercial Specialty | Property Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 3,234 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 748,185 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2019 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | 76,844 | 73,968 | 69,114 | $ 69,145 |  |  |  |  |  |
IBNR | [2] | $ 9,924 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 2,695 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 88,347 | 84,260 | 83,553 |  |  |  |  |  |  |
IBNR | [2] | $ 23,282 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 2,719 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 113,382 | 112,124 |  |  |  |  |  |  |  |
IBNR | [2] | $ 55,202 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 3,689 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 138,225 |  |  |  |  |  |  |  |  |
IBNR | [2] | $ 106,457 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 2,782 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2013 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 56,336 | 56,786 | 56,901 | 58,961 | $ 61,714 | $ 65,193 | $ 66,566 | $ 68,081 | $ 68,230 | $ 63,931
IBNR | [2] | $ 1,226 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 2,597 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2014 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 49,134 | 50,919 | 51,329 | 54,971 | 57,230 | 57,828 | 58,618 | 60,779 | 61,427 |
IBNR | [2] | $ 2,006 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 2,400 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2015 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 55,322 | 56,279 | 56,498 | 60,231 | 58,993 | 58,384 | 57,088 | 57,710 |  |
IBNR | [2] | $ 2,652 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 2,162 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2016 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 52,347 | 52,401 | 52,760 | 52,078 | 53,751 | 54,123 | 54,576 |  |  |
IBNR | [2] | $ 2,949 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 1,997 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2017 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 57,369 | 56,273 | 54,704 | 53,876 | 54,978 | 54,654 |  |  |  |
IBNR | [2] | $ 4,535 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 1,915 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2018 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 60,879 | 60,372 | 57,922 | 57,605 | 58,220 |  |  |  |  |
IBNR | [2] | $ 7,436 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 2,345 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 192,016 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2019 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | 14,080 | 13,685 | 13,686 | 13,686 |  |  |  |  |  |
IBNR | [3] | $ 11,092 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 31,258 | 30,375 | 30,398 |  |  |  |  |  |  |
IBNR | [3] | $ 23,998 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 50,122 | 49,177 |  |  |  |  |  |  |  |
IBNR | [3] | $ 42,927 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 86,624 |  |  |  |  |  |  |  |  |
IBNR | [3] | $ 82,247 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2013 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 0 | 0 | 0 | 0 | 850 | 850 | 850 | 1,009 | 1,009 | $ 1,009
IBNR | [3] | $ 0 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2014 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 0 | 0 | 0 | 590 | 1,954 | 1,954 | 1,954 | 1,987 | $ 1,987 |
IBNR | [3] | $ 0 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2015 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 0 | 0 | 1,090 | 2,179 | 2,779 | 2,779 | 2,779 | 2,779 |  |
IBNR | [3] | $ 0 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2016 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 0 | 3,627 | 3,627 | 3,627 | 3,627 | 3,627 | 3,627 |  |  |
IBNR | [3] | $ 0 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2017 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 4,358 | 4,358 | 4,358 | 4,358 | 4,358 | 4,358 |  |  |  |
IBNR | [3] | $ 4,356 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2018 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 5,574 | 5,574 | 5,573 | 5,573 | 5,573 |  |  |  |  |
IBNR | [3] | $ 5,568 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 471,113 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance | Accident Year 2019 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | 160,865 | 162,523 | 159,713 | 155,116 |  |  |  |  |  |
IBNR | [4] | $ 6,645 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 161,351 | 159,718 | 155,747 |  |  |  |  |  |  |
IBNR | [4] | $ 9,558 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 106,079 | 108,708 |  |  |  |  |  |  |  |
IBNR | [4] | $ 4,715 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 42,818 |  |  |  |  |  |  |  |  |
IBNR | [4] | $ 4,017 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 115,152 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2019 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | 13,613 | 13,870 | 13,518 | 13,574 |  |  |  |  |  |
IBNR | [5] | $ 2,934 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 13,697 | 13,099 | 13,285 |  |  |  |  |  |  |
IBNR | [5] | $ 4,555 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 12,160 | 12,376 |  |  |  |  |  |  |  |
IBNR | [5] | $ 8,115 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 8,317 |  |  |  |  |  |  |  |  |
IBNR | [5] | $ 5,986 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2015 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 19,837 | 20,045 | 20,677 | 21,133 | 21,720 | 21,243 | 20,170 | $ 18,612 |  |
IBNR | [5] | $ 864 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2016 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 16,074 | 16,256 | 16,719 | 17,035 | 19,423 | 20,726 | $ 21,030 |  |  |
IBNR | [5] | $ 1,127 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2017 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 15,114 | 15,516 | 17,056 | 17,624 | 18,730 | $ 19,683 |  |  |  |
IBNR | [5] | $ 1,518 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2018 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Allocated Claims Adjustment Expenses, Net of Reinsurance |  | $ 16,340 | $ 15,979 | $ 15,414 | $ 15,439 | $ 14,835 |  |  |  |  |
IBNR | [5] | $ 1,721 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | Claim |  | 0 |  |  |  |  |  |  |  |  |
[1] | Incurred-but-not-reported liabilities plus expected development on reported claims
[2] | Incurred-but-not-reported liabilities plus expected development on reported claims
[3] | Incurred-but-not-reported liabilities plus expected development on reported claims
[4] | Incurred-but-not-reported liabilities plus expected development on reported claims
[5] | Incurred-but-not-reported liabilities plus expected development on reported claims

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