Investments - Schedule of Fair Value of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 2,839.1	$ 2,590.6
U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	343.0	477.8
Unrealized gains	0.0	6.6
Unrealized losses	(6.7)	(0.2)
Fair value	336.3	484.2
Agencies
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost		6.9
Unrealized gains		0.0
Unrealized losses		0.0
Fair value		6.9
Non-U.S. government
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	39.1	46.6
Unrealized gains	0.0	0.6
Unrealized losses	(0.5)	0.0
Fair value	38.6	47.2
Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,730.7	1,616.8
Unrealized gains	1.5	35.7
Unrealized losses	(34.3)	(0.3)
Fair value	1,697.9	1,652.2
Residential mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	428.8	244.3
Unrealized gains	0.1	5.6
Unrealized losses	(4.5)	0.0
Fair value	424.4	249.9
Commercial mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	10.3	1.0
Unrealized gains	0.0	0.0
Unrealized losses	(0.2)	0.0
Fair value	10.1	1.0
Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	287.2	197.2
Unrealized gains	0.3	1.9
Unrealized losses	(2.1)	(0.1)
Fair value	285.4	199.0
Fixed maturity securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	2,839.1	2,590.6
Unrealized gains	1.9	50.4
Unrealized losses	(48.3)	(0.6)
Fair value	2,792.7	2,640.4
Fixed maturity securities \| U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Fair value	336.3	484.2
Fixed maturity securities \| Agencies
Debt Securities, Available-for-Sale [Line Items]
Fair value		6.9
Fixed maturity securities \| Non-U.S. government
Debt Securities, Available-for-Sale [Line Items]
Fair value	38.6	47.2
Fixed maturity securities \| Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Fair value	1,697.9	1,652.2
Fixed maturity securities \| Residential mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Fair value	424.4	249.9
Fixed maturity securities \| Commercial mortgage-backed
Debt Securities, Available-for-Sale [Line Items]
Fair value	10.1	1.0
Fixed maturity securities \| Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Fair value	285.4	199.0
Short-term investments
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	269.1	111.3
Unrealized gains	0.0	0.0
Unrealized losses	(0.4)	0.0
Fair value	268.7	111.3
Short-term investments \| U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	267.3	109.9
Unrealized gains	0.0	0.0
Unrealized losses	(0.4)	0.0
Fair value	266.9	109.9
Short-term investments \| Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1.4	1.0
Unrealized gains	0.0	0.0
Unrealized losses	0.0	0.0
Fair value	1.4	1.0
Short-term investments \| Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	0.4	0.4
Unrealized gains	0.0	0.0
Unrealized losses	0.0	0.0
Fair value	$ 0.4	$ 0.4

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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