Investments - Allowance for Credit Losses (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, December 31, 2021	$ 2.5	$ 6.6
Securities for which allowance was not previously recorded	0.5	2.1
Securities sold during the period	(0.6)	(0.4)
Additional net increases (decreases) in existing allowance	0.5	(1.1)
Ending balance, March 31, 2022	2.9	7.2
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, December 31, 2021	0.2	0.2
Securities for which allowance was not previously recorded	0.1	0.0
Securities sold during the period	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	(0.1)
Ending balance, March 31, 2022	0.3	0.1
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, December 31, 2021	0.0	0.1
Securities for which allowance was not previously recorded	0.0	0.0
Securities sold during the period	0.0	0.0
Additional net increases (decreases) in existing allowance	0.4	(0.1)
Ending balance, March 31, 2022	0.4	0.0
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, December 31, 2021	2.2	6.1
Securities for which allowance was not previously recorded	0.4	2.1
Securities sold during the period	(0.6)	(0.4)
Additional net increases (decreases) in existing allowance	0.1	(0.7)
Ending balance, March 31, 2022	2.1	7.1
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, December 31, 2021	0.1	0.2
Securities for which allowance was not previously recorded	0.0	0.0
Securities sold during the period	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	(0.2)
Ending balance, March 31, 2022	$ 0.1	$ 0.0