Business and Significant Accounting Policies - Schedule of Intangible Assets and Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Finite-Lived Intangible Assets [Line Items]
Total other gross carrying amount	$ 185.8
Gross Carrying Amount	362.1
Other accumulated amortization	(46.5)
Accumulated Amortization	(209.9)
Net Value	127.2
Other intangible assets, net	139.3	$ 180.6
Total intangible assets	152.2
Insurance licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible asset carrying value	25.0
Value of Business Acquired
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	176.3
Accumulated Amortization	(163.4)
Net Value	12.9
Broker relations
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	21.5
Accumulated Amortization	(1.7)
Net Value	$ 19.8
Broker relations | Minimum
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life	13 years
Broker relations | Maximum
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life	16 years
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 12.9
Accumulated Amortization	(2.9)
Net Value	$ 10.0
Weighted average useful life	5 years
Leases
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 7.1
Accumulated Amortization	(0.8)
Net Value	$ 6.3
Weighted average useful life	9 years
Internally developed software
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 16.5
Accumulated Amortization	(3.1)
Net Value	$ 13.4
Internally developed software | Minimum
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life	5 years
Internally developed software | Maximum
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life	7 years
Fair Value Adjustment For Loss And LAE Reserves
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 102.8
Accumulated Amortization	(38.0)
Net Value	$ 64.8