Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Fair value, less than one year	$ 117.0	$ 107.2
Unrealized losses, less than one year	2.8	1.4
Fair value, one year or greater	9.7	0.0
Unrealized losses, one year or greater	0.3	0.0
Fair value, total	126.7	107.2
Unrealized losses, total	3.1	1.4
U.S. Governments
Schedule of Investments [Line Items]
Fair value, less than one year	17.4	0.0
Unrealized losses, less than one year	0.6	0.0
Fair value, one year or greater	0.0	0.0
Unrealized losses, one year or greater	0.0	0.0
Fair value, total	17.4	0.0
Unrealized losses, total	0.6	0.0
Foreign Governments
Schedule of Investments [Line Items]
Fair value, less than one year		0.1
Unrealized losses, less than one year		0.0
Fair value, one year or greater		0.0
Unrealized losses, one year or greater		0.0
Fair value, total		0.1
Unrealized losses, total		0.0
Obligations of states and political subdivisions
Schedule of Investments [Line Items]
Fair value, less than one year		0.5
Unrealized losses, less than one year		0.0
Fair value, one year or greater		0.0
Unrealized losses, one year or greater		0.0
Fair value, total		0.5
Unrealized losses, total		0.0
Corporate bonds
Schedule of Investments [Line Items]
Fair value, less than one year	19.6	38.7
Unrealized losses, less than one year	1.2	0.8
Fair value, one year or greater	1.7	0.0
Unrealized losses, one year or greater	0.1	0.0
Fair value, total	21.3	38.7
Unrealized losses, total	1.3	0.8
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	9.7	32.2
Unrealized losses, less than one year	0.1	0.5
Fair value, one year or greater	8.0	0.0
Unrealized losses, one year or greater	0.2	0.0
Fair value, total	17.7	32.2
Unrealized losses, total	0.3	0.5
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	48.9	2.9
Unrealized losses, less than one year	0.6	0.0
Fair value, one year or greater	0.0	0.0
Unrealized losses, one year or greater	0.0	0.0
Fair value, total	48.9	2.9
Unrealized losses, total	0.6	0.0
Asset-backed securities
Schedule of Investments [Line Items]
Fair value, less than one year	9.9	11.4
Unrealized losses, less than one year	0.1	0.1
Fair value, one year or greater	0.0	0.0
Unrealized losses, one year or greater	0.0	0.0
Fair value, total	9.9	11.4
Unrealized losses, total	0.1	0.1
Collateralized loan obligations
Schedule of Investments [Line Items]
Fair value, less than one year	11.5	21.4
Unrealized losses, less than one year	0.2	0.0
Fair value, one year or greater	0.0	0.0
Unrealized losses, one year or greater	0.0	0.0
Fair value, total	11.5	21.4
Unrealized losses, total	$ 0.2	$ 0.0