v3.26.1
Investment Securities - Investment Holdings (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2026
Dec. 31, 2025
U.S. Government and agency obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost $ 480.1 $ 459.7
Gross Unrealized Gains 0.5 1.3
Gross Unrealized Losses (2.9) (0.7)
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 477.7 460.3
Debt securities issued by corporations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 1,640.1 1,534.9
Gross Unrealized Gains 5.9 13.5
Gross Unrealized Losses (18.5) (11.5)
Net Foreign Currency Gains (Losses) 1.3 0.8
Carrying Value 1,628.8 1,537.7
Mortgage and asset-backed securities    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 417.2 405.1
Gross Unrealized Gains 2.3 4.2
Gross Unrealized Losses (17.0) (16.3)
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 402.5 393.0
Collateralized loan obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 344.7 343.9
Gross Unrealized Gains 0.3 0.8
Gross Unrealized Losses (0.6) (0.1)
Net Foreign Currency Gains (Losses) 0.8 2.4
Carrying Value 345.2 347.0
Foreign government and agency obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 31.1 31.2
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.3) (0.1)
Net Foreign Currency Gains (Losses) 1.1 1.4
Carrying Value 31.9 32.5
Fixed maturity investments    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 2,913.2 2,774.8
Gross Unrealized Gains 9.0 19.8
Gross Unrealized Losses (39.3) (28.7)
Net Foreign Currency Gains (Losses) 3.2 4.6
Carrying Value $ 2,886.1 $ 2,770.5