Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) - Fixed maturities - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Fair value, less than one year	$ 1,967.8	$ 488.2
Unrealized losses, less than one year	36.9	9.8
Fair value, one year or greater	70.0	69.7
Unrealized losses, one year or greater	3.9	7.3
Fair Value	2,037.8	557.9
Unrealized Losses	40.8	17.1
U.S. Governments
Schedule of Investments [Line Items]
Fair value, less than one year	193.4	40.6
Unrealized losses, less than one year	2.6	0.3
Fair value, one year or greater	14.6	0.0
Unrealized losses, one year or greater	0.6	0.0
Fair Value	208.0	40.6
Unrealized Losses	3.2	0.3
Foreign Governments
Schedule of Investments [Line Items]
Fair value, less than one year	152.4	18.0
Unrealized losses, less than one year	3.3	0.5
Fair value, one year or greater	2.6	0.1
Unrealized losses, one year or greater	0.6	0.2
Fair Value	155.0	18.1
Unrealized Losses	3.9	0.7
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Fair value, less than one year	46.0	5.2
Unrealized losses, less than one year	0.8	0.3
Fair value, one year or greater	0.1	0.0
Unrealized losses, one year or greater	0.4	0.0
Fair Value	46.1	5.2
Unrealized Losses	1.2	0.3
Corporate bonds
Schedule of Investments [Line Items]
Fair value, less than one year	854.3	202.5
Unrealized losses, less than one year	18.3	6.7
Fair value, one year or greater	41.7	17.5
Unrealized losses, one year or greater	2.0	6.6
Fair Value	896.0	220.0
Unrealized Losses	20.3	13.3
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	198.8	21.8
Unrealized losses, less than one year	4.1	0.3
Fair value, one year or greater	6.5	0.0
Unrealized losses, one year or greater	0.2	0.0
Fair Value	205.3	21.8
Unrealized Losses	4.3	0.3
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	284.2	74.4
Unrealized losses, less than one year	5.6	0.4
Fair value, one year or greater	4.0	3.0
Unrealized losses, one year or greater	0.1	0.2
Fair Value	288.2	77.4
Unrealized Losses	5.7	0.6
Asset backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	62.6	4.6
Unrealized losses, less than one year	0.6	0.4
Fair value, one year or greater	0.0	0.0
Unrealized losses, one year or greater		0.0
Fair Value	62.6	4.6
Unrealized Losses	0.6	0.4
Collateralized loan obligations
Schedule of Investments [Line Items]
Fair value, less than one year	176.1	121.1
Unrealized losses, less than one year	1.6	0.9
Fair value, one year or greater	0.5	49.1
Unrealized losses, one year or greater		0.3
Fair Value	176.6	170.2
Unrealized Losses	$ 1.6	$ 1.2