v3.26.1
Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details)
$ in Millions
Dec. 31, 2025
USD ($)
security
Dec. 31, 2024
USD ($)
Security
security
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 746.6 $ 1,625.0
Gross unrealized losses, less than twelve months (3.7) (22.3)
Over 12 months, Fair Market Value 748.2 1,604.9
Over 12 months, Gross Unrealized Losses (54.5) (159.6)
Total, Fair Market Value 1,494.8 3,229.9
Total, Gross Unrealized Losses $ (58.2) $ (181.9)
Number of Securities 606 1,170
Securities Below Investment Grade or NR $ (0.3)  
Fixed income securities — Available for sale    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value   $ 161.2
Gross unrealized losses, less than twelve months   (2.2)
Over 12 months, Fair Market Value   351.3
Over 12 months, Gross Unrealized Losses   (79.9)
Total, Fair Market Value   512.5
Total, Gross Unrealized Losses   $ (82.1)
Number of Securities | Security   225
U.S. government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value 152.2 $ 718.1
Gross unrealized losses, less than twelve months (1.2) (11.6)
Over 12 months, Fair Market Value 123.1 397.8
Over 12 months, Gross Unrealized Losses (3.3) (14.1)
Total, Fair Market Value 275.3 1,115.9
Total, Gross Unrealized Losses $ (4.5) $ (25.7)
Number of Securities | security 43 137
Municipal    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 5.5 $ 0.0
Gross unrealized losses, less than twelve months 0.0 0.0
Over 12 months, Fair Market Value 14.1 7.2
Over 12 months, Gross Unrealized Losses (0.2) (0.3)
Total, Fair Market Value 19.6 7.2
Total, Gross Unrealized Losses $ (0.2) $ (0.3)
Number of Securities | security 14 1
Corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 253.1 $ 33.6
Gross unrealized losses, less than twelve months (1.2) (0.7)
Over 12 months, Fair Market Value 387.7 45.4
Over 12 months, Gross Unrealized Losses (22.9) (1.2)
Total, Fair Market Value 640.8 79.0
Total, Gross Unrealized Losses $ (24.1) $ (1.9)
Number of Securities | security 302 47
Non-U.S. government-backed corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 7.7 $ 570.3
Gross unrealized losses, less than twelve months (0.1) (6.8)
Over 12 months, Fair Market Value 23.7 663.9
Over 12 months, Gross Unrealized Losses (0.4) (59.6)
Total, Fair Market Value 31.4 1,234.2
Total, Gross Unrealized Losses $ (0.5) $ (66.4)
Number of Securities | security 5 692
Non-U.S. government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 89.2 $ 5.8
Gross unrealized losses, less than twelve months (0.6) 0.0
Over 12 months, Fair Market Value 17.4 91.1
Over 12 months, Gross Unrealized Losses (0.5) (2.4)
Total, Fair Market Value 106.6 96.9
Total, Gross Unrealized Losses $ (1.1) $ (2.4)
Number of Securities | security 40 12
Non-agency commercial mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 0.0 $ 17.2
Gross unrealized losses, less than twelve months 0.0 (0.1)
Over 12 months, Fair Market Value 4.7 0.0
Over 12 months, Gross Unrealized Losses (0.3) 0.0
Total, Fair Market Value 4.7 17.2
Total, Gross Unrealized Losses $ (0.3) $ (0.1)
Number of Securities | security 1 14
Agency residential mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 71.4  
Gross unrealized losses, less than twelve months (0.1)  
Over 12 months, Fair Market Value 177.5  
Over 12 months, Gross Unrealized Losses (26.9)  
Total, Fair Market Value 248.9  
Total, Gross Unrealized Losses $ (27.0)  
Number of Securities | security 141  
Agency mortgage-backed    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value   $ 0.0
Gross unrealized losses, less than twelve months   0.0
Over 12 months, Fair Market Value   4.5
Over 12 months, Gross Unrealized Losses   (0.6)
Total, Fair Market Value   4.5
Total, Gross Unrealized Losses   $ (0.6)
Number of Securities | security   1
Asset-backed    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 167.5 $ 118.8
Gross unrealized losses, less than twelve months (0.5) (0.9)
Over 12 months, Fair Market Value 0.0 43.7
Over 12 months, Gross Unrealized Losses 0.0 (1.5)
Total, Fair Market Value 167.5 162.5
Total, Gross Unrealized Losses $ (0.5) $ (2.4)
Number of Securities | security 60 41