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)