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Supplementary Information about Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Detail) - USD ($) $ in Thousands | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | $ 735,088 |  |  |  |  |  |  |  |  |
Commercial Specialty | Property Insurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 198,244 |  |  |  |  |  |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 47,500 |  |  |  |  |  |  |  |  |
Commercial Specialty | Property Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 73,640 | $ 69,944 | $ 47,486 |  |  |  |  |  |  |
Commercial Specialty | Property Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 74,860 | 50,846 |  |  |  |  |  |  |  |
Commercial Specialty | Property Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 49,744 |  |  |  |  |  |  |  |  |
Commercial Specialty | Property Insurance | All outstanding liabilities before 2020, net of reinsurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
All outstanding liabilities, net of reinsurance | 3,025 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 412,063 |  |  |  |  |  |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 380,628 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 53,525 | 53,471 | 53,194 | $ 52,468 | $ 51,167 | $ 46,475 | $ 38,641 | $ 29,705 | $ 17,969 | $ 6,439
Commercial Specialty | Casualty Insurance | Accident Year 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 46,578 | 46,263 | 45,831 | 43,505 | 40,293 | 34,504 | 26,526 | 15,924 | 4,011 |
Commercial Specialty | Casualty Insurance | Accident Year 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 50,169 | 48,524 | 46,272 | 43,116 | 36,357 | 25,559 | 14,865 | 3,355 |  |
Commercial Specialty | Casualty Insurance | Accident Year 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 47,352 | 44,130 | 40,639 | 34,973 | 22,064 | 14,047 | 4,148 |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 46,169 | 39,926 | 33,511 | 23,326 | 12,879 | 4,996 |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 42,884 | 34,745 | 22,194 | 13,869 | 4,303 |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 48,285 | 30,439 | 14,017 | 5,222 |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 40,665 | 19,623 | 5,510 |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 24,414 | 7,238 |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 12,022 |  |  |  |  |  |  |  |  |
Commercial Specialty | Casualty Insurance | All outstanding liabilities before 2013, net of reinsurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
All outstanding liabilities, net of reinsurance | 44,506 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 12,408 |  |  |  |  |  |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 179,608 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance |  |  | 0 | 0 | 0 |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 0 | 0 |  |  |  | 0 | 0 | 0 | 0 | $ 0
Reinsurance Operations | Casualty Insurance | Accident Year 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | $ 0 |
Reinsurance Operations | Casualty Insurance | Accident Year 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 0 | 0 | 0 | 0 | 0 | 0 | 0 |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 2 | 2 | 2 | 2 | 2 | 0 |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 6 | 6 | 0 | 0 | 0 |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 1,925 | 1,014 | 801 | 27 |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 4,503 | 2,174 | 48 |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 3,986 | 1,593 |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 1,986 |  |  |  |  |  |  |  |  |
Reinsurance Operations | Casualty Insurance | All outstanding liabilities before 2013, net of reinsurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
All outstanding liabilities, net of reinsurance | 0 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 426,907 |  |  |  |  |  |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 89,018 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance | Accident Year 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 145,406 | 140,463 | 128,338 | 99,554 |  |  |  |  |  |
Exited Lines Segment | Property Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 148,159 | 143,332 | 121,430 |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 98,809 | 82,528 |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 34,533 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Property Insurance | All outstanding liabilities before 2019, net of reinsurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
All outstanding liabilities, net of reinsurance | 44,812 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 81,501 |  |  |  |  |  |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 38,334 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 18,806 | 18,785 | 18,584 | 17,959 | 15,575 | 12,515 | 8,605 | $ 3,527 |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 14,783 | 14,347 | 12,478 | 11,824 | 9,745 | 6,865 | $ 3,494 |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 12,413 | 12,021 | 11,585 | 8,406 | 4,635 | $ 2,051 |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 14,169 | 12,593 | 8,840 | 4,522 | $ 1,334 |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 9,820 | 7,900 | 5,011 | $ 1,975 |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 7,531 | 4,949 | $ 1,583 |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 3,125 | $ 1,628 |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | Accident Year 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 854 |  |  |  |  |  |  |  |  |
Exited Lines Segment | Casualty Insurance | All outstanding liabilities before 2015, net of reinsurance |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
All outstanding liabilities, net of reinsurance | $ 4,683 |  |  |  |  |  |  |  |  |

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