Investments - Schedule of Allowance for Credit Losses (Detail) - USD ($) $ in Millions	1 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 0.0	$ 2.8	$ 0.2
Securities for which allowance was not previously recorded	0.0	2.7	0.8
Securities sold during the period	0.0	(0.7)	(0.1)
Reductions for credit impairments	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.2	(0.5)	0.0
Ending balance	0.2	4.3	0.9
Mortgage And Private Loans
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.2	0.2	0.2
Securities for which allowance was not previously recorded	0.0	0.0	5.6
Securities sold during the period	0.0	0.0	(0.4)
Reductions for credit impairments	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.0	0.2
Ending balance	0.2	0.2	5.6
Mortgage Loan
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.2	0.2	0.2
Securities for which allowance was not previously recorded	0.0	0.0	0.8
Securities sold during the period	0.0	0.0	(0.4)
Reductions for credit impairments	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.0	0.6
Ending balance	0.2	0.2	1.2
Private Loans
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.0	0.0
Securities for which allowance was not previously recorded	0.0	0.0	4.8
Securities sold during the period	0.0	0.0	0.0
Reductions for credit impairments	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.0	(0.4)
Ending balance	0.0	0.0	4.4
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0		0.0
Ending balance	0.0
Cumulative Effect, Period of Adoption, Adjustment | Mortgage And Private Loans
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.0	0.0
Ending balance	0.0
Cumulative Effect, Period of Adoption, Adjustment | Mortgage Loan
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.0	0.0
Ending balance	0.0
Cumulative Effect, Period of Adoption, Adjustment | Private Loans
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.0	0.0
Ending balance	0.0
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.7	0.0
Securities for which allowance was not previously recorded	0.0	0.1	0.0
Securities sold during the period	0.0	0.0	0.0
Reductions for credit impairments	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.3	0.0
Ending balance	0.0	1.1	0.0
Foreign Governments | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0		0.0
Ending balance	0.0
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.4	0.0
Securities for which allowance was not previously recorded	0.0	0.5	0.0
Securities sold during the period	0.0	0.0	0.0
Reductions for credit impairments	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	(0.4)	0.0
Ending balance	0.0	0.5	0.0
Obligations of states and political subdivisions | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0		0.0
Ending balance	0.0
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	1.6	0.2
Securities for which allowance was not previously recorded	0.0	2.1	0.8
Securities sold during the period	0.0	(0.7)	(0.1)
Reductions for credit impairments	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.2	(0.4)	0.0
Ending balance	0.2	2.6	0.9
Corporate bonds | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0		0.0
Ending balance	0.0
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.1	0.0
Securities for which allowance was not previously recorded	0.0	0.0	0.0
Securities sold during the period	0.0	0.0	0.0
Reductions for credit impairments	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.0	0.0
Ending balance	0.0	$ 0.1	0.0
Asset-backed securities | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0		$ 0.0
Ending balance	$ 0.0