v3.26.3
Fair Value Measurements - Schedule of Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Assets    
Investments pending settlement $ 19.5 $ 4.8
Derivative assets 0.3 2.4
Total other assets 19.8 7.2
Total assets measured at fair value 3,081.2 2,758.9
Liabilities    
Investments pending settlement (21.3) (6.6)
Derivative liabilities   (1.7)
Total other liabilities (21.3) (8.3)
Total liabilities measured at fair value (21.3) (8.3)
U.S. Treasuries    
Assets    
Fair value 336.3 484.2
Agencies    
Assets    
Fair value   6.9
Non-U.S. government    
Assets    
Fair value 38.6 47.2
Corporate bonds    
Assets    
Fair value 1,697.9 1,652.2
Residential mortgage-backed    
Assets    
Fair value 424.4 249.9
Commercial mortgage-backed    
Assets    
Fair value 10.1 1.0
Other asset-backed securities    
Assets    
Fair value 285.4 199.0
Fixed maturity securities    
Assets    
Fair value 2,792.7 2,640.4
Fixed maturity securities | U.S. Treasuries    
Assets    
Fair value 336.3 484.2
Fixed maturity securities | Agencies    
Assets    
Fair value   6.9
Fixed maturity securities | Non-U.S. government    
Assets    
Fair value 38.6 47.2
Fixed maturity securities | Corporate bonds    
Assets    
Fair value 1,697.9 1,652.2
Fixed maturity securities | Residential mortgage-backed    
Assets    
Fair value 424.4 249.9
Fixed maturity securities | Commercial mortgage-backed    
Assets    
Fair value 10.1 1.0
Fixed maturity securities | Other asset-backed securities    
Assets    
Fair value 285.4 199.0
Short-term investments    
Assets    
Fair value 268.7 111.3
Short-term investments | U.S. Treasuries    
Assets    
Fair value 266.9 109.9
Short-term investments | Corporate bonds    
Assets    
Fair value 1.4 1.0
Short-term investments | Other asset-backed securities    
Assets    
Fair value 0.4 0.4
Level 1    
Assets    
Investments pending settlement 19.5 4.8
Derivative assets 0.0 0.0
Total other assets 19.5 4.8
Total assets measured at fair value 622.7 598.9
Liabilities    
Investments pending settlement (21.3) (6.6)
Derivative liabilities   0.0
Total other liabilities (21.3) (6.6)
Total liabilities measured at fair value (21.3) (6.6)
Level 1 | Fixed maturity securities    
Assets    
Fair value 336.3 484.2
Level 1 | Fixed maturity securities | U.S. Treasuries    
Assets    
Fair value 336.3 484.2
Level 1 | Fixed maturity securities | Agencies    
Assets    
Fair value   0.0
Level 1 | Fixed maturity securities | Non-U.S. government    
Assets    
Fair value 0.0 0.0
Level 1 | Fixed maturity securities | Corporate bonds    
Assets    
Fair value 0.0 0.0
Level 1 | Fixed maturity securities | Residential mortgage-backed    
Assets    
Fair value 0.0 0.0
Level 1 | Fixed maturity securities | Commercial mortgage-backed    
Assets    
Fair value 0.0 0.0
Level 1 | Fixed maturity securities | Other asset-backed securities    
Assets    
Fair value 0.0 0.0
Level 1 | Short-term investments    
Assets    
Fair value 266.9 109.9
Level 1 | Short-term investments | U.S. Treasuries    
Assets    
Fair value 266.9 109.9
Level 1 | Short-term investments | Corporate bonds    
Assets    
Fair value 0.0 0.0
Level 1 | Short-term investments | Other asset-backed securities    
Assets    
Fair value 0.0 0.0
Level 2    
Assets    
Investments pending settlement 0.0 0.0
Derivative assets 0.3 2.4
Total other assets 0.3 2.4
Total assets measured at fair value 2,458.5 2,160.0
Liabilities    
Investments pending settlement 0.0 0.0
Derivative liabilities   (1.7)
Total other liabilities 0.0 (1.7)
Total liabilities measured at fair value 0.0 (1.7)
Level 2 | Fixed maturity securities    
Assets    
Fair value 2,456.4 2,156.2
Level 2 | Fixed maturity securities | U.S. Treasuries    
Assets    
Fair value 0.0 0.0
Level 2 | Fixed maturity securities | Agencies    
Assets    
Fair value   6.9
Level 2 | Fixed maturity securities | Non-U.S. government    
Assets    
Fair value 38.6 47.2
Level 2 | Fixed maturity securities | Corporate bonds    
Assets    
Fair value 1,697.9 1,652.2
Level 2 | Fixed maturity securities | Residential mortgage-backed    
Assets    
Fair value 424.4 249.9
Level 2 | Fixed maturity securities | Commercial mortgage-backed    
Assets    
Fair value 10.1 1.0
Level 2 | Fixed maturity securities | Other asset-backed securities    
Assets    
Fair value 285.4 199.0
Level 2 | Short-term investments    
Assets    
Fair value 1.8 1.4
Level 2 | Short-term investments | U.S. Treasuries    
Assets    
Fair value 0.0 0.0
Level 2 | Short-term investments | Corporate bonds    
Assets    
Fair value 1.4 1.0
Level 2 | Short-term investments | Other asset-backed securities    
Assets    
Fair value 0.4 0.4
Level 3    
Assets    
Investments pending settlement 0.0 0.0
Derivative assets 0.0 0.0
Total other assets 0.0 0.0
Total assets measured at fair value 0.0 0.0
Liabilities    
Investments pending settlement 0.0 0.0
Derivative liabilities   0.0
Total other liabilities 0.0 0.0
Total liabilities measured at fair value 0.0 0.0
Level 3 | Fixed maturity securities    
Assets    
Fair value 0.0 0.0
Level 3 | Fixed maturity securities | U.S. Treasuries    
Assets    
Fair value 0.0 0.0
Level 3 | Fixed maturity securities | Agencies    
Assets    
Fair value   0.0
Level 3 | Fixed maturity securities | Non-U.S. government    
Assets    
Fair value 0.0 0.0
Level 3 | Fixed maturity securities | Corporate bonds    
Assets    
Fair value 0.0 0.0
Level 3 | Fixed maturity securities | Residential mortgage-backed    
Assets    
Fair value 0.0 0.0
Level 3 | Fixed maturity securities | Commercial mortgage-backed    
Assets    
Fair value 0.0 0.0
Level 3 | Fixed maturity securities | Other asset-backed securities    
Assets    
Fair value 0.0 0.0
Level 3 | Short-term investments    
Assets    
Fair value 0.0 0.0
Level 3 | Short-term investments | U.S. Treasuries    
Assets    
Fair value 0.0 0.0
Level 3 | Short-term investments | Corporate bonds    
Assets    
Fair value 0.0 0.0
Level 3 | Short-term investments | Other asset-backed securities    
Assets    
Fair value $ 0.0 $ 0.0