v3.26.3
Investment Securities - Investment Holdings (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2026
Dec. 31, 2025
U.S. Government and agency obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost $ 459.3 $ 459.7
Gross Unrealized Gains 0.0 1.3
Gross Unrealized Losses (10.8) (0.7)
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 448.5 460.3
Debt securities issued by corporations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 1,837.4 1,534.9
Gross Unrealized Gains 0.4 13.5
Gross Unrealized Losses (57.5) (11.5)
Net Foreign Currency Gains (Losses) 1.2 0.8
Carrying Value 1,781.5 1,537.7
Mortgage and asset-backed securities    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 429.5 405.1
Gross Unrealized Gains 0.0 4.2
Gross Unrealized Losses (36.7) (16.3)
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 392.8 393.0
Collateralized loan obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 379.4 343.9
Gross Unrealized Gains 0.5 0.8
Gross Unrealized Losses (0.1) (0.1)
Net Foreign Currency Gains (Losses) (1.4) 2.4
Carrying Value 378.4 347.0
Foreign government and agency obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 38.7 31.2
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.3) (0.1)
Net Foreign Currency Gains (Losses) 1.2 1.4
Carrying Value 39.6 32.5
Fixed maturity investments    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 3,144.3 2,774.8
Gross Unrealized Gains 0.9 19.8
Gross Unrealized Losses (105.4) (28.7)
Net Foreign Currency Gains (Losses) 1.0 4.6
Carrying Value $ 3,040.8 $ 2,770.5