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Comprehensive Income - Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2025 | Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Income tax expense (benefit) |  | $ (5,211) | $ (924) | $ (989)
Net of tax and noncontrolling interest |  | (261,692) | (556,449) | 3,632
Total before tax |  | (95,803) | (58,921) | (23,232)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 96,848 | (9,750) |
Realized Investment Gains (Losses) |  | (2,577) | (496) | $ 19
Unrealized Gains (Losses) on Available-for-Sale Securities [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Net realized investment gains |  |  | (3,919) |
Net of tax and noncontrolling interest |  | (3,615) | (4,522) |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | (3,615) | (4,522) |
Realized Investment Gains (Losses) |  | 15,193 |  |
Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 1,109 | (356) |
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, before Tax |  | 590 | (474) |
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax |  | (1,109) | (356) |
Accumulated Translation Adjustment [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 99,354 | 0 |
Accumulated Translation Adjustment [Member] | Discontinued Operations, Disposed of by Sale | Ambac Assurance Corporation |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Realized Investment Gains (Losses) |  | (14,957) |  |
Disposal Group, Including Discontinued Operation, Foreign Currency Translation Gains (Losses) | $ 99,354 |  |  |
Credit Risk Changes of Fair Value, Net Loss on Discontinued Operation |  | 698 |  |
Reclassification out of Accumulated Other Comprehensive Income [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Net of tax and noncontrolling interest |  | 96,848 | (9,817) |
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Available-for-Sale Securities [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Income tax expense (benefit) |  | 3,851 | 603 |
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, Tax |  | (179) |  |
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Financial Liability, Fair Value Option, Including Portion Attributable to Noncontrolling Interest [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, Tax |  |  | 118 |
Other Postretirement Benefits Plan [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 0 | (4,872) |
Other Postretirement Benefits Plan [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] |  |  |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Income tax expense (benefit) |  | 0 | 0 |
Prior service cost |  | 0 | (210) |
Actuarial gains (losses) |  | 0 | (67) |
Total before tax |  | 0 | (4,939) |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 0 | (4,939) |
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), after Tax |  | $ 0 | $ (4,662) |

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