Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	$ 6,026.7	$ 5,842.5
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	6,026.7	5,842.5
Gross Unrealized Gains	74.1	48.4
Gross Unrealized Losses	386.2	503.0
Fair Value	5,714.6	5,387.9
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	718.1	827.9
Gross Unrealized Gains	9.5	2.7
Gross Unrealized Losses	38.6	74.8
Fair Value	689.0	755.8
Other, including U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	381.4	426.5
Gross Unrealized Gains	0.4	0.1
Gross Unrealized Losses	56.1	69.0
Fair Value	325.7	357.6
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,235.3	1,239.1
Gross Unrealized Gains	24.6	17.5
Gross Unrealized Losses	82.7	105.8
Fair Value	1,177.2	1,150.8
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	10.6	14.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.6	1.0
Fair Value	10.0	13.1
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	2,064.8	1,995.2
Gross Unrealized Gains	27.6	17.3
Gross Unrealized Losses	182.1	230.1
Fair Value	1,910.3	1,782.4
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,616.5	1,339.7
Gross Unrealized Gains	12.0	10.8
Gross Unrealized Losses	26.1	22.3
Fair Value	$ 1,602.4	$ 1,328.2