Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Schedule of Investments [Line Items]
Amortized Cost, net	$ 5,857.7	$ 5,842.5
Fair Value	5,457.7	5,387.9
Fixed maturity securities
Schedule of Investments [Line Items]
Amortized Cost, net	5,857.7	5,842.5
Gross Unrealized Gains	53.5	48.4
Gross Unrealized Losses	453.5	503.0
Fair Value	5,457.7	5,387.9
Mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	866.8	827.9
Gross Unrealized Gains	6.1	2.7
Gross Unrealized Losses	60.7	74.8
Fair Value	812.2	755.8
Other, including U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost, net	422.5	426.5
Gross Unrealized Gains	0.4	0.1
Gross Unrealized Losses	59.6	69.0
Fair Value	363.3	357.6
Municipal bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,236.1	1,239.1
Gross Unrealized Gains	18.2	17.5
Gross Unrealized Losses	98.4	105.8
Fair Value	1,155.9	1,150.8
Foreign government bonds
Schedule of Investments [Line Items]
Amortized Cost, net	14.1	14.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.8	1.0
Fair Value	13.3	13.1
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,972.2	1,995.2
Gross Unrealized Gains	20.1	17.3
Gross Unrealized Losses	212.4	230.1
Fair Value	1,779.9	1,782.4
Other asset-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	1,346.0	1,339.7
Gross Unrealized Gains	8.7	10.8
Gross Unrealized Losses	21.6	22.3
Fair Value	$ 1,333.1	$ 1,328.2