Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Schedule of Investments [Line Items]
Amortized Cost, net	$ 5,985.5	$ 5,842.5
Fair Value	5,666.5	5,387.9
Fixed maturity securities
Schedule of Investments [Line Items]
Amortized Cost, net	5,985.5	5,842.5
Gross Unrealized Gains	76.2	48.4
Gross Unrealized Losses	395.2	503.0
Fair Value	5,666.5	5,387.9
Mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	663.9	827.9
Gross Unrealized Gains	8.1	2.7
Gross Unrealized Losses	39.6	74.8
Fair Value	632.4	755.8
Other, including U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost, net	415.5	426.5
Gross Unrealized Gains	1.0	0.1
Gross Unrealized Losses	56.6	69.0
Fair Value	359.9	357.6
Municipal bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,264.0	1,239.1
Gross Unrealized Gains	22.2	17.5
Gross Unrealized Losses	88.0	105.8
Fair Value	1,198.2	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	0.6	1.0
Fair Value	13.5	13.1
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,989.9	1,995.2
Gross Unrealized Gains	32.4	17.3
Gross Unrealized Losses	181.6	230.1
Fair Value	1,840.7	1,782.4
Other asset-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	1,638.1	1,339.7
Gross Unrealized Gains	12.5	10.8
Gross Unrealized Losses	28.8	22.3
Fair Value	$ 1,621.8	$ 1,328.2