v3.26.1
Investment Securities - Investment Holdings (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2026
Dec. 31, 2025
U.S. Government and agency obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost $ 445.5 $ 459.7
Gross Unrealized Gains 0.1 1.3
Gross Unrealized Losses (2.9) (0.7)
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 442.7 460.3
Debt securities issued by corporations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 1,818.0 1,534.9
Gross Unrealized Gains 4.3 13.5
Gross Unrealized Losses (19.0) (11.5)
Net Foreign Currency Gains (Losses) 1.4 0.8
Carrying Value 1,804.7 1,537.7
Mortgage and asset-backed securities    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 426.8 405.1
Gross Unrealized Gains 1.7 4.2
Gross Unrealized Losses (18.2) (16.3)
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 410.3 393.0
Collateralized loan obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 337.2 343.9
Gross Unrealized Gains 0.6 0.8
Gross Unrealized Losses 0.0 (0.1)
Net Foreign Currency Gains (Losses) (0.1) 2.4
Carrying Value 337.7 347.0
Foreign government and agency obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 43.4 31.2
Gross Unrealized Gains 0.1 0.0
Gross Unrealized Losses (0.1) (0.1)
Net Foreign Currency Gains (Losses) 1.1 1.4
Carrying Value 44.5 32.5
Fixed maturity investments    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 3,070.9 2,774.8
Gross Unrealized Gains 6.8 19.8
Gross Unrealized Losses (40.2) (28.7)
Net Foreign Currency Gains (Losses) 2.4 4.6
Carrying Value $ 3,039.9 $ 2,770.5