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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Fixed maturities
Amortized Cost	$ 4,302.7	$ 4,203.2
Allowance for Credit Losses	2.9	2.5	$ 7.2	$ 6.6
Fair Value	4,143.9	4,223.3
Fixed maturities
Fixed maturities
Amortized Cost	4,302.7	4,203.2
Gross Unrealized Gains	19.5	63.4
Gross Unrealized Losses	175.4	40.8
Allowance for Credit Losses	2.9	2.5
Fair Value	4,143.9	4,223.3
Fixed maturities \| U.S. Governments
Fixed maturities
Amortized Cost	496.0	422.7
Gross Unrealized Gains	0.4	5.5
Gross Unrealized Losses	17.0	3.2
Allowance for Credit Losses	0.0	0.0
Fair Value	479.4	425.0
Fixed maturities \| Foreign Governments
Fixed maturities
Amortized Cost	211.7	234.7
Gross Unrealized Gains	3.7	2.2
Gross Unrealized Losses	7.6	3.9
Allowance for Credit Losses	0.3	0.2
Fair Value	207.5	232.8
Fixed maturities \| Obligations of states and political subdivisions
Fixed maturities
Amortized Cost	169.6	166.7
Gross Unrealized Gains	2.3	5.8
Gross Unrealized Losses	4.9	1.2
Allowance for Credit Losses	0.4	0.0
Fair Value	166.6	171.3
Fixed maturities \| Corporate bonds
Fixed maturities
Amortized Cost	2,009.0	1,972.3
Gross Unrealized Gains	9.7	33.5
Gross Unrealized Losses	86.2	20.3
Allowance for Credit Losses	2.1	2.2
Fair Value	1,930.4	1,983.3
Fixed maturities \| Commercial mortgage-backed securities
Fixed maturities
Amortized Cost	422.7	416.7
Gross Unrealized Gains	0.6	6.3
Gross Unrealized Losses	23.1	4.3
Allowance for Credit Losses	0.0	0.0
Fair Value	400.2	418.7
Fixed maturities \| Residential mortgage-backed securities
Fixed maturities
Amortized Cost	455.4	480.7
Gross Unrealized Gains	1.9	7.5
Gross Unrealized Losses	26.5	5.7
Allowance for Credit Losses	0.0	0.0
Fair Value	430.8	482.5
Fixed maturities \| Asset-backed securities
Fixed maturities
Amortized Cost	205.6	173.0
Gross Unrealized Gains	0.3	1.3
Gross Unrealized Losses	5.3	0.6
Allowance for Credit Losses	0.1	0.1
Fair Value	200.5	173.6
Fixed maturities \| Collateralized loan obligations
Fixed maturities
Amortized Cost	332.7	336.4
Gross Unrealized Gains	0.6	1.3
Gross Unrealized Losses	4.8	1.6
Allowance for Credit Losses	0.0	0.0
Fair Value	$ 328.5	$ 336.1