Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Fixed maturities
Amortized Cost	$ 2,969.8	$ 3,016.4
Gross Unrealized Gains	2.7	2.1
Gross Unrealized Losses	305.4	340.2
Allowance for Credit Losses	2.7	2.8	$ 2.9	$ 2.5
Fair Value	2,664.4	2,675.5
U.S. Governments
Fixed maturities
Amortized Cost	412.5	410.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	24.6	30.2
Allowance for Credit Losses	0.0	0.0
Fair Value	387.9	380.7
Foreign Governments
Fixed maturities
Amortized Cost	35.8	35.6
Gross Unrealized Gains	0.4	0.3
Gross Unrealized Losses	7.2	6.7
Allowance for Credit Losses	0.8	0.8
Fair Value	28.2	28.4
Obligations of states and political subdivisions
Fixed maturities
Amortized Cost	109.0	109.9
Gross Unrealized Gains	0.6	0.4
Gross Unrealized Losses	8.3	10.1
Allowance for Credit Losses	0.4	0.4
Fair Value	100.9	99.8
Corporate bonds
Fixed maturities
Amortized Cost	1,360.5	1,394.8
Gross Unrealized Gains	1.1	0.9
Gross Unrealized Losses	140.7	160.0
Allowance for Credit Losses	1.4	1.6
Fair Value	1,219.5	1,234.1
Commercial mortgage-backed securities
Fixed maturities
Amortized Cost	335.2	337.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	50.4	52.0
Allowance for Credit Losses	0.0	0.0
Fair Value	284.8	285.4
Residential mortgage-backed securities
Fixed maturities
Amortized Cost	314.1	320.0
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	46.1	50.2
Allowance for Credit Losses	0.0	0.0
Fair Value	268.2	270.0
Asset-backed securities
Fixed maturities
Amortized Cost	149.3	153.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	12.1	14.2
Allowance for Credit Losses	0.1	0.0
Fair Value	137.1	139.2
Collateralized loan obligations
Fixed maturities
Amortized Cost	253.4	254.4
Gross Unrealized Gains	0.4	0.3
Gross Unrealized Losses	16.0	16.8
Allowance for Credit Losses	0.0	0.0
Fair Value	$ 237.8	$ 237.9