v3.26.1
Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities and Short-Term Investments (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost $ 5,106.0 $ 5,147.0
Gross Unrealized Gains 61.8 14.2
Gross Unrealized Losses (58.2) (181.9)
Allowance for Credit Losses (0.3) (1.0)
Fair Market Value 5,109.3 4,978.3
Fixed income investments    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 5,011.0 4,861.1
Gross Unrealized Gains 60.9 14.0
Gross Unrealized Losses (58.2) (181.9)
Allowance for Credit Losses (0.3) (1.0)
Fair Market Value 5,013.4 4,692.2
U.S. government    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 1,206.4 1,504.8
Gross Unrealized Gains 20.1 1.5
Gross Unrealized Losses (4.5) (25.7)
Allowance for Credit Losses 0.0 0.0
Fair Market Value 1,222.0 1,480.6
U.S. agency    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 2.0 7.5
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 (0.3)
Allowance for Credit Losses 0.0 0.0
Fair Market Value 2.0 7.2
Municipal    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 90.9 84.2
Gross Unrealized Gains 1.6 0.0
Gross Unrealized Losses (0.2) (1.9)
Allowance for Credit Losses 0.0 0.0
Fair Market Value 92.3 82.3
Corporate    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 1,995.2 2,045.3
Gross Unrealized Gains 26.8 8.5
Gross Unrealized Losses (24.1) (66.4)
Allowance for Credit Losses (0.3) (1.0)
Fair Market Value 1,997.6 1,986.4
Non-U.S. government-backed corporate    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 66.0 132.9
Gross Unrealized Gains 0.8 0.8
Gross Unrealized Losses (0.5) (2.4)
Allowance for Credit Losses 0.0 0.0
Fair Market Value 66.3 131.3
Non-U.S. government    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 400.6 248.2
Gross Unrealized Gains 2.1 1.0
Gross Unrealized Losses (1.1) (2.4)
Allowance for Credit Losses 0.0 0.0
Fair Market Value 401.6 246.8
Asset-backed    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 499.5 232.4
Gross Unrealized Gains 2.0 2.2
Gross Unrealized Losses (0.5) (0.1)
Allowance for Credit Losses 0.0 0.0
Fair Market Value 501.0 234.5
Agency commercial mortgage-backed securities    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 9.8 5.0
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.3) (0.6)
Allowance for Credit Losses 0.0 0.0
Fair Market Value 9.5 4.4
Agency residential mortgage-backed securities    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 740.6 600.8
Gross Unrealized Gains 7.5 0.0
Gross Unrealized Losses (27.0) (82.1)
Allowance for Credit Losses 0.0 0.0
Fair Market Value 721.1 518.7
Short-term investments — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 45.4 261.9
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Allowance for Credit Losses 0.0 0.0
Fair Market Value 45.4 261.9
Asset backed securities, privately-held afs    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 15.8 24.0
Gross Unrealized Gains 0.5 0.2
Gross Unrealized Losses 0.0 0.0
Allowance for Credit Losses 0.0 0.0
Fair Market Value 16.3 $ 24.2
Privately-held investments, global corporate securities    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 33.8  
Gross Unrealized Gains 0.4  
Gross Unrealized Losses 0.0  
Allowance for Credit Losses 0.0  
Fair Market Value 34.2  
Total, privately-held investments, AFS    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 49.6  
Gross Unrealized Gains 0.9  
Gross Unrealized Losses 0.0  
Allowance for Credit Losses 0.0  
Fair Market Value $ 50.5