Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	$ 6,125.5	$ 6,026.7
Fixed maturity securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	6,125.5	6,026.7
Gross Unrealized Gains	52.0	74.1
Gross Unrealized Losses	410.5	386.2
Fair Value	5,767.0	5,714.6
Mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	699.0	718.1
Gross Unrealized Gains	5.4	9.5
Gross Unrealized Losses	42.2	38.6
Fair Value	662.2	689.0
Other, including U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	371.8	381.4
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	58.6	56.1
Fair Value	313.2	325.7
Municipal bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	1,222.4	1,235.3
Gross Unrealized Gains	23.9	24.6
Gross Unrealized Losses	82.8	82.7
Fair Value	1,163.5	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	2,139.1	2,064.8
Gross Unrealized Gains	15.2	27.6
Gross Unrealized Losses	199.8	182.1
Fair Value	1,954.5	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	7.5	12.0
Gross Unrealized Losses	26.4	26.1
Fair Value	$ 1,665.7	$ 1,602.4

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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