Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Fixed maturities
Amortized Cost	$ 2,370.0	$ 2,521.5
Gross Unrealized Gains	45.3	65.5
Gross Unrealized Losses	1.7	1.4
Allowance for Credit Losses	0.2	0.2	$ 2.7	$ 2.8
Fair value	2,413.4	2,585.4
U.S. Governments
Fixed maturities
Amortized Cost	354.4	357.7
Gross Unrealized Gains	1.6	4.5
Gross Unrealized Losses	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair value	356.0	362.2
Foreign Governments
Fixed maturities
Amortized Cost	5.6	27.9
Gross Unrealized Gains	0.6	2.6
Gross Unrealized Losses	0.0	0.0
Allowance for Credit Losses	0.0	0.2
Fair value	6.2	30.3
Obligations of states and political subdivisions
Fixed maturities
Amortized Cost	87.2	92.4
Gross Unrealized Gains	1.5	2.0
Gross Unrealized Losses	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair value	88.7	94.4
Corporate bonds
Fixed maturities
Amortized Cost	1,097.4	1,185.0
Gross Unrealized Gains	19.2	28.9
Gross Unrealized Losses	0.9	0.8
Allowance for Credit Losses	0.2	0.0
Fair value	1,115.5	1,213.1
Commercial mortgage-backed securities
Fixed maturities
Amortized Cost	264.5	270.9
Gross Unrealized Gains	11.4	10.1
Gross Unrealized Losses	0.5	0.5
Allowance for Credit Losses	0.0	0.0
Fair value	275.4	280.5
Residential mortgage-backed securities
Fixed maturities
Amortized Cost	230.7	235.2
Gross Unrealized Gains	6.1	13.6
Gross Unrealized Losses	0.0	0.0
Allowance for Credit Losses	0.0	0.0
Fair value	236.8	248.8
Asset-backed securities
Fixed maturities
Amortized Cost	131.9	140.4
Gross Unrealized Gains	1.9	1.7
Gross Unrealized Losses	0.1	0.1
Allowance for Credit Losses	0.0	0.0
Fair value	133.7	142.0
Collateralized loan obligations
Fixed maturities
Amortized Cost	198.3	212.0
Gross Unrealized Gains	3.0	2.1
Gross Unrealized Losses	0.2	0.0
Allowance for Credit Losses	0.0	0.0
Fair value	$ 201.1	$ 214.1