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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Fixed maturities
Amortized Cost	$ 4,156.5		$ 4,203.2
Gross Unrealized Gains	5.0		63.4
Gross Unrealized Losses	310.5		40.8
Allowance for Credit Losses	2.0	$ 2.9	2.5	$ 6.4	$ 7.2	$ 6.6
Fair Value	3,849.0		4,223.3
U.S. Governments
Fixed maturities
Amortized Cost	511.5		422.7
Gross Unrealized Gains	0.0		5.5
Gross Unrealized Losses	23.2		3.2
Allowance for Credit Losses	0.0		0.0
Fair Value	488.3		425.0
Foreign Governments
Fixed maturities
Amortized Cost	209.2		234.7
Gross Unrealized Gains	0.4		2.2
Gross Unrealized Losses	15.2		3.9
Allowance for Credit Losses	0.5		0.2
Fair Value	193.9		232.8
Obligations of states and political subdivisions
Fixed maturities
Amortized Cost	178.6		166.7
Gross Unrealized Gains	1.3		5.8
Gross Unrealized Losses	7.8		1.2
Allowance for Credit Losses	0.4		0.0
Fair Value	171.7		171.3
Corporate bonds
Fixed maturities
Amortized Cost	1,894.3		1,972.3
Gross Unrealized Gains	2.6		33.5
Gross Unrealized Losses	152.6		20.3
Allowance for Credit Losses	1.0		2.2
Fair Value	1,743.3		1,983.3
Commercial mortgage-backed securities
Fixed maturities
Amortized Cost	407.7		416.7
Gross Unrealized Gains	0.0		6.3
Gross Unrealized Losses	40.6		4.3
Allowance for Credit Losses	0.0		0.0
Fair Value	367.1		418.7
Residential mortgage-backed securities
Fixed maturities
Amortized Cost	442.4		480.7
Gross Unrealized Gains	0.6		7.5
Gross Unrealized Losses	43.6		5.7
Allowance for Credit Losses	0.0		0.0
Fair Value	399.4		482.5
Asset-backed securities
Fixed maturities
Amortized Cost	202.7		173.0
Gross Unrealized Gains	0.0		1.3
Gross Unrealized Losses	10.6		0.6
Allowance for Credit Losses	0.1		0.1
Fair Value	192.0		173.6
Collateralized loan obligations
Fixed maturities
Amortized Cost	310.1		336.4
Gross Unrealized Gains	0.1		1.3
Gross Unrealized Losses	16.9		1.6
Allowance for Credit Losses	0.0		0.0
Fair Value	$ 293.3		$ 336.1