Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	$ 6,147.4	$ 6,026.7
Fixed maturity securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	6,147.4	6,026.7
Gross Unrealized Gains	56.5	74.1
Gross Unrealized Losses	423.8	386.2
Fair Value	5,780.1	5,714.6
Mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	703.2	718.1
Gross Unrealized Gains	6.5	9.5
Gross Unrealized Losses	40.6	38.6
Fair Value	669.1	689.0
Other, including U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	370.9	381.4
Gross Unrealized Gains	0.1	0.4
Gross Unrealized Losses	58.1	56.1
Fair Value	312.9	325.7
Municipal bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	1,240.5	1,235.3
Gross Unrealized Gains	20.9	24.6
Gross Unrealized Losses	90.2	82.7
Fair Value	1,171.2	1,177.2
Foreign government bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	8.6	10.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.7	0.6
Fair Value	7.9	10.0
Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	2,139.6	2,064.8
Gross Unrealized Gains	18.2	27.6
Gross Unrealized Losses	204.3	182.1
Fair Value	1,953.5	1,910.3
Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	1,684.6	1,616.5
Gross Unrealized Gains	10.8	12.0
Gross Unrealized Losses	29.9	26.1
Fair Value	$ 1,665.5	$ 1,602.4