v3.26.1
Investments - Schedule of Fair Value of Investments (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost $ 2,845.6 $ 2,590.6
U.S. Treasuries    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 420.5 477.8
Unrealized gains 1.5 6.6
Unrealized losses (1.1) (0.2)
Fair value 420.9 484.2
Agencies    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 3.8 6.9
Unrealized gains 0.0 0.0
Unrealized losses 0.0 0.0
Fair value 3.8 6.9
Non-U.S. government    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 38.5 46.6
Unrealized gains 0.1 0.6
Unrealized losses (0.1) 0.0
Fair value 38.5 47.2
Corporate bonds    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 1,699.3 1,616.8
Unrealized gains 12.9 35.7
Unrealized losses (2.9) (0.3)
Fair value 1,709.3 1,652.2
Residential mortgage-backed    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 402.2 244.3
Unrealized gains 2.3 5.6
Unrealized losses (0.1) 0.0
Fair value 404.4 249.9
Commercial mortgage-backed    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 9.5 1.0
Unrealized gains 0.0 0.0
Unrealized losses 0.0 0.0
Fair value 9.5 1.0
Other asset-backed securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 271.8 197.2
Unrealized gains 0.8 1.9
Unrealized losses (0.5) (0.1)
Fair value 272.1 199.0
Fixed maturity securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 2,845.6 2,590.6
Unrealized gains 17.6 50.4
Unrealized losses (4.7) (0.6)
Fair value 2,858.5 2,640.4
Fixed maturity securities | U.S. Treasuries    
Debt Securities, Available-for-Sale [Line Items]    
Fair value 420.9 484.2
Fixed maturity securities | Agencies    
Debt Securities, Available-for-Sale [Line Items]    
Fair value 3.8 6.9
Fixed maturity securities | Non-U.S. government    
Debt Securities, Available-for-Sale [Line Items]    
Fair value 38.5 47.2
Fixed maturity securities | Corporate bonds    
Debt Securities, Available-for-Sale [Line Items]    
Fair value 1,709.3 1,652.2
Fixed maturity securities | Residential mortgage-backed    
Debt Securities, Available-for-Sale [Line Items]    
Fair value 404.4 249.9
Fixed maturity securities | Commercial mortgage-backed    
Debt Securities, Available-for-Sale [Line Items]    
Fair value 9.5 1.0
Fixed maturity securities | Other asset-backed securities    
Debt Securities, Available-for-Sale [Line Items]    
Fair value 272.1 199.0
Short-term investments    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 255.0 111.3
Unrealized gains 0.0 0.0
Unrealized losses (0.2) 0.0
Fair value 254.8 111.3
Short-term investments | U.S. Treasuries    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 253.8 109.9
Unrealized gains 0.0 0.0
Unrealized losses (0.2) 0.0
Fair value 253.6 109.9
Short-term investments | Corporate bonds    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 1.2 1.0
Unrealized gains 0.0 0.0
Unrealized losses 0.0 0.0
Fair value $ 1.2 1.0
Short-term investments | Other asset-backed securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost   0.4
Unrealized gains   0.0
Unrealized losses   0.0
Fair value   $ 0.4