v3.25.1
Investment Securities - Investment Holdings (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2025
Dec. 31, 2024
U.S. Government and agency obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost $ 489.0 $ 461.8
Gross Unrealized Gains 2.3 1.1
Gross Unrealized Losses (0.4) (0.8)
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 490.9 462.1
Debt securities issued by corporations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 1,415.5 1,444.5
Gross Unrealized Gains 8.2 4.6
Gross Unrealized Losses (22.3) (31.4)
Net Foreign Currency Gains (Losses) (2.9) (3.5)
Carrying Value 1,398.5 1,414.2
Municipal obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 3.2 3.2
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 3.2 3.2
Mortgage and asset-backed securities    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 399.6 400.2
Gross Unrealized Gains 1.3 0.2
Gross Unrealized Losses (21.0) (26.5)
Net Foreign Currency Gains (Losses) 0.0 0.0
Carrying Value 379.9 373.9
Collateralized loan obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 256.7 237.3
Gross Unrealized Gains 0.4 1.2
Gross Unrealized Losses (0.6) (0.2)
Net Foreign Currency Gains (Losses) (0.5) (1.6)
Carrying Value 256.0 236.7
Foreign government and agency obligations    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 23.4 22.2
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 (0.1)
Net Foreign Currency Gains (Losses) 0.1 (0.6)
Carrying Value 23.5 21.5
Fixed maturity investments    
Debt and Equity Securities, FV-NI [Line Items]    
Cost or Amortized Cost 2,587.4 2,569.2
Gross Unrealized Gains 12.2 7.1
Gross Unrealized Losses (44.3) (59.0)
Net Foreign Currency Gains (Losses) (3.3) (5.7)
Carrying Value $ 2,552.0 $ 2,511.6