Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	$ 6,149.9	$ 6,026.7
Fixed maturity securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	6,149.9	6,026.7
Gross Unrealized Gains	17.7	74.1
Gross Unrealized Losses	646.8	386.2
Fair Value	5,520.8	5,714.6
Mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	726.5	718.1
Gross Unrealized Gains	0.3	9.5
Gross Unrealized Losses	74.7	38.6
Fair Value	652.1	689.0
Other, including U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	376.9	381.4
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	79.0	56.1
Fair Value	297.9	325.7
Municipal bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	1,216.5	1,235.3
Gross Unrealized Gains	9.4	24.6
Gross Unrealized Losses	146.0	82.7
Fair Value	1,079.9	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	1.0	0.6
Fair Value	7.6	10.0
Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	2,126.3	2,064.8
Gross Unrealized Gains	3.9	27.6
Gross Unrealized Losses	295.0	182.1
Fair Value	1,835.2	1,910.3
Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost, net	1,695.1	1,616.5
Gross Unrealized Gains	4.1	12.0
Gross Unrealized Losses	51.1	26.1
Fair Value	$ 1,648.1	$ 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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