Investments - Fair Value of Investments (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 2,590.6	$ 3,403.8
Fixed maturity securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	2,590.6	3,403.8
Unrealized gains	50.4	16.6
Unrealized losses	(0.6)	(8.8)
Fair value	2,640.4	3,411.6
Fixed maturity securities \| U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	477.8	747.6
Unrealized gains	6.6	2.1
Unrealized losses	(0.2)	(3.1)
Fair value	484.2	746.6
Fixed maturity securities \| Agencies
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	6.9	11.5
Unrealized gains	0.0	0.0
Unrealized losses	0.0	0.0
Fair value	6.9	11.5
Fixed maturity securities \| Non-U.S. government
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	46.6	46.6
Unrealized gains	0.6	0.1
Unrealized losses	0.0	(0.3)
Fair value	47.2	46.4
Fixed maturity securities \| Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,616.8	1,906.3
Unrealized gains	35.7	10.9
Unrealized losses	(0.3)	(4.0)
Fair value	1,652.2	1,913.2
Fixed maturity securities \| Residential mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	244.3	279.5
Unrealized gains	5.6	0.8
Unrealized losses	0.0	(1.2)
Fair value	249.9	279.1
Fixed maturity securities \| Commercial mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1.0	0.0
Unrealized gains	0.0	0.4
Unrealized losses	0.0	0.0
Fair value	1.0	0.4
Fixed maturity securities \| Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	197.2	412.3
Unrealized gains	1.9	2.3
Unrealized losses	(0.1)	(0.2)
Fair value	199.0	414.4
Short-term investments
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	111.3	221.9
Unrealized gains	0.0	0.2
Unrealized losses	0.0	0.0
Fair value	111.3	222.1
Short-term investments \| U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	109.9	220.5
Unrealized gains	0.0	0.2
Unrealized losses	0.0	0.0
Fair value	109.9	220.7
Short-term investments \| Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1.0	1.1
Unrealized gains	0.0	0.0
Unrealized losses	0.0	0.0
Fair value	1.0	1.1
Short-term investments \| Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	0.4	0.3
Unrealized gains	0.0	0.0
Unrealized losses	0.0	0.0
Fair value	$ 0.4	$ 0.3

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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