Investments - Allowance for Credit Losses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 6.6	$ 0.0
Securities for which allowance was not previously recorded	2.9	15.9
Securities sold during the period	(3.5)	(39.3)
Additional net increases (decreases) in existing allowance	(3.5)	23.1
Ending balance	2.5	6.6
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		6.9
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.2	0.0
Securities for which allowance was not previously recorded	0.0	0.3
Securities sold during the period	0.0	(0.2)
Additional net increases (decreases) in existing allowance	0.0	0.1
Ending balance	0.2	0.2
Foreign Governments | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		0.0
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.1	0.0
Securities for which allowance was not previously recorded	0.0	0.3
Securities sold during the period	0.0	0.0
Additional net increases (decreases) in existing allowance	(0.1)	(0.2)
Ending balance	0.0	0.1
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
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.1	0.0
Securities for which allowance was not previously recorded	2.7	15.3
Securities sold during the period	(3.5)	(39.0)
Additional net increases (decreases) in existing allowance	(3.1)	23.0
Ending balance	2.2	6.1
Corporate bonds | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		6.8
Asset backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.2	0.0
Securities for which allowance was not previously recorded	0.2	0.0
Securities sold during the period	0.0	(0.1)
Additional net increases (decreases) in existing allowance	(0.3)	0.2
Ending balance	$ 0.1	0.2
Asset backed securities | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance		$ 0.1