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, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Fair value, less than one year	$ 1,557.5	$ 1,967.8
Unrealized losses, less than one year	143.6	36.9
Fair value, one year or greater	1,066.4	70.0
Unrealized losses, one year or greater	196.6	3.9
Fair value, total	2,623.9	2,037.8
Unrealized losses, total	340.2	40.8
U.S. Governments
Schedule of Investments [Line Items]
Fair value, less than one year	271.0	193.4
Unrealized losses, less than one year	18.1	2.6
Fair value, one year or greater	109.8	14.6
Unrealized losses, one year or greater	12.1	0.6
Fair value, total	380.8	208.0
Unrealized losses, total	30.2	3.2
Foreign Governments
Schedule of Investments [Line Items]
Fair value, less than one year	16.7	152.4
Unrealized losses, less than one year	4.9	3.3
Fair value, one year or greater	2.6	2.6
Unrealized losses, one year or greater	1.8	0.6
Fair value, total	19.3	155.0
Unrealized losses, total	6.7	3.9
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Fair value, less than one year	67.4	46.0
Unrealized losses, less than one year	4.1	0.8
Fair value, one year or greater	24.3	0.1
Unrealized losses, one year or greater	6.0	0.4
Fair value, total	91.7	46.1
Unrealized losses, total	10.1	1.2
Corporate bonds
Schedule of Investments [Line Items]
Fair value, less than one year	695.1	854.3
Unrealized losses, less than one year	68.3	18.3
Fair value, one year or greater	519.6	41.7
Unrealized losses, one year or greater	91.7	2.0
Fair value, total	1,214.7	896.0
Unrealized losses, total	160.0	20.3
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	144.2	198.8
Unrealized losses, less than one year	18.6	4.1
Fair value, one year or greater	141.2	6.5
Unrealized losses, one year or greater	33.4	0.2
Fair value, total	285.4	205.3
Unrealized losses, total	52.0	4.3
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	88.7	284.2
Unrealized losses, less than one year	8.8	5.6
Fair value, one year or greater	178.8	4.0
Unrealized losses, one year or greater	41.4	0.1
Fair value, total	267.5	288.2
Unrealized losses, total	50.2	5.7
Asset-backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	93.3	62.6
Unrealized losses, less than one year	7.5	0.6
Fair value, one year or greater	45.9	0.0
Unrealized losses, one year or greater	6.7	0.0
Fair value, total	139.2	62.6
Unrealized losses, total	14.2	0.6
Collateralized loan obligations
Schedule of Investments [Line Items]
Fair value, less than one year	181.1	176.1
Unrealized losses, less than one year	13.3	1.6
Fair value, one year or greater	44.2	0.5
Unrealized losses, one year or greater	3.5	0.0
Fair value, total	225.3	176.6
Unrealized losses, total	$ 16.8	$ 1.6