Investments - Allowance for Credit Losses (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 2.8	$ 2.5
Securities for which allowance was not previously recorded	0.2	0.5
Securities sold during the period	(0.3)	(0.6)
Reductions for credit impairments	0.0
Additional net increases (decreases) in existing allowance	0.0	0.5
Ending balance	2.7	2.9
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.7	0.2
Securities for which allowance was not previously recorded	0.0	0.1
Securities sold during the period	0.0	0.0
Reductions for credit impairments	0.0
Additional net increases (decreases) in existing allowance	0.1	0.0
Ending balance	0.8	0.3
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.4	0.0
Securities for which allowance was not previously recorded	0.0	0.0
Securities sold during the period	0.0	0.0
Reductions for credit impairments	0.0
Additional net increases (decreases) in existing allowance	0.0	0.4
Ending balance	0.4	0.4
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.6	2.2
Securities for which allowance was not previously recorded	0.2	0.4
Securities sold during the period	(0.3)	(0.6)
Reductions for credit impairments	0.0
Additional net increases (decreases) in existing allowance	(0.1)	0.1
Ending balance	1.4	2.1
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.1	0.1
Securities for which allowance was not previously recorded	0.0	0.0
Securities sold during the period	0.0	0.0
Reductions for credit impairments	0.0
Additional net increases (decreases) in existing allowance	0.0	0.0
Ending balance	$ 0.1	$ 0.1