Business and Significant Accounting Policies - Intangible Assets and Accumulated Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Nov. 16, 2023	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Total other gross carrying amount	$ 185.8
Gross Carrying Amount	362.1
Other accumulated amortization	(5.2)
Accumulated Amortization	(37.9)
Net Value	299.2
Other intangible assets, net	180.6		$ 0.0
Total intangible assets	324.2	$ 362.1	$ 0.0
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	(32.7)
Net Value	143.6
Broker relations
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	21.5
Accumulated Amortization	(0.2)
Net Value	$ 21.3
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	(0.3)
Net Value	$ 12.6
Weighted average useful life	5 years
Leases
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 7.1
Accumulated Amortization	(0.1)
Net Value	$ 7.0
Weighted average useful life	9 years
Internally developed software
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 16.5
Accumulated Amortization	(0.3)
Net Value	$ 16.2
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	(4.3)
Net Value	$ 98.5