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Investments - Debt Securities by Contractual Maturity (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Debt securities, AFS - Cost or amortized cost    
Due in one year or less $ 450.6 $ 449.3
Due after one year through five years 1,411.9 1,648.0
Due after five years through ten years 533.0 589.5
Due after ten years 117.7 104.3
Cost or amortized cost 4,617.0 5,143.8
Debt securities, AFS - Fair value    
Due in one year or less 449.5 448.2
Due after one year through five years 1,419.7 1,646.7
Due after five years through ten years 538.3 586.2
Due after ten years 115.0 101.9
Debt securities, available for sale 4,635.2 5,131.0
Debt securities, trading - Cost or Amortized cost    
Due in one year or less 0.7 0.5
Due after one year through five years 6.3 6.7
Due after five years through ten years 0.2 0.3
Due after ten years 12.6 11.9
Cost or amortized cost 140.9 187.3
Debt securities, trading - Fair value    
Due in one year or less 0.7 0.5
Due after one year through five years 6.0 6.3
Due after five years through ten years 0.2 0.3
Due after ten years 1.1 1.6
Debt securities, trading 117.6 162.2
Mortgage-backed and asset-backed securities    
Debt securities, AFS - Cost or amortized cost    
Without single maturity date 2,103.8 2,352.7
Debt securities, AFS - Fair value    
Without single maturity date 2,112.7 2,348.0
Debt securities, trading - Cost or Amortized cost    
Without single maturity date 121.1 167.9
Debt securities, trading - Fair value    
Without single maturity date $ 109.6 $ 153.5