Investments - Allowance for Credit Losses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	$ 6.4	$ 40.2	$ 6.6	$ 0.0
Securities for which allowance was not previously recorded	0.9	13.7	2.9	15.0
Securities sold during the period	(0.2)	(14.9)	(1.1)	(15.7)
Additional net increases (decreases) in existing allowance	(0.1)	(5.3)	(1.4)	27.5
Ending balance, September 30, 2021	7.0	33.7	7.0	33.7
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Additions-initial adoption of accounting standard					$ 6.9
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	0.2	0.3	0.2	0.0
Additions-initial adoption of accounting standard					0.0
Securities for which allowance was not previously recorded	0.0	0.1	0.0	0.3
Securities sold during the period	0.0	(0.2)	0.0	(0.2)
Additional net increases (decreases) in existing allowance	0.0	0.0	0.0	0.1
Ending balance, September 30, 2021	0.2	0.2	0.2	0.2
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	0.0	0.1	0.1	0.0
Additions-initial adoption of accounting standard					0.0
Securities for which allowance was not previously recorded	0.0	0.0	0.0	0.3
Securities sold during the period	0.0	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.1	(0.1)	(0.1)
Ending balance, September 30, 2021	0.0	0.2	0.0	0.2
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	6.2	39.7	6.1	0.0
Additions-initial adoption of accounting standard					6.8
Securities for which allowance was not previously recorded	0.7	13.6	2.7	14.4
Securities sold during the period	(0.2)	(14.7)	(1.1)	(15.5)
Additional net increases (decreases) in existing allowance	(0.1)	(6.4)	(1.1)	26.5
Ending balance, September 30, 2021	6.6	32.2	6.6	32.2
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, June 30, 2021	0.0	0.1	0.2	0.0
Additions-initial adoption of accounting standard					$ 0.1
Securities for which allowance was not previously recorded	0.2	0.0	0.2	0.0
Securities sold during the period	0.0	0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	1.0	(0.2)	1.0
Ending balance, September 30, 2021	$ 0.2	$ 1.1	$ 0.2	$ 1.1