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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details)
$ in Millions
Dec. 31, 2024
USD ($)
security
Dec. 31, 2023
USD ($)
security
Security
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 1,625.0 $ 196.5
Gross unrealized losses, less than twelve months (22.3) (2.1)
Over 12 months, Fair Market Value 1,604.9 2,802.0
Over 12 months, Gross Unrealized Losses (159.6) (219.7)
Total, Fair Market Value 3,229.9 2,998.5
Total, Gross Unrealized Losses $ (181.9) $ (221.8)
Number of Securities 1,170 950
Securities Below Investment Grade or NR   $ 1.2
Fixed income securities — Available for sale    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value   0.1
Gross unrealized losses, less than twelve months   0.0
Over 12 months, Fair Market Value   435.9
Over 12 months, Gross Unrealized Losses   (74.9)
Total, Fair Market Value   436.0
Total, Gross Unrealized Losses   $ (74.9)
Number of Securities | Security   197
U.S. government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 718.1 $ 105.5
Gross unrealized losses, less than twelve months (11.6) (0.5)
Over 12 months, Fair Market Value 397.8 673.3
Over 12 months, Gross Unrealized Losses (14.1) (26.2)
Total, Fair Market Value 1,115.9 778.8
Total, Gross Unrealized Losses $ (25.7) $ (26.7)
Number of Securities | security 137 74
U.S. agency    
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 7.2  
Over 12 months, Gross Unrealized Losses (0.3)  
Total, Fair Market Value 7.2  
Total, Gross Unrealized Losses $ (0.3)  
Number of Securities | security 1  
Municipal    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 33.6 $ 0.0
Gross unrealized losses, less than twelve months (0.7) 0.0
Over 12 months, Fair Market Value 45.4 7.2
Over 12 months, Gross Unrealized Losses (1.2) (0.3)
Total, Fair Market Value 79.0 7.2
Total, Gross Unrealized Losses $ (1.9) $ (0.3)
Number of Securities | security 47 1
Corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 570.3 $ 11.1
Gross unrealized losses, less than twelve months (6.8) (1.0)
Over 12 months, Fair Market Value 663.9 117.0
Over 12 months, Gross Unrealized Losses (59.6) (4.0)
Total, Fair Market Value 1,234.2 128.1
Total, Gross Unrealized Losses $ (66.4) $ (5.0)
Number of Securities | security 692 54
Non-U.S. government-backed corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 5.8 $ 46.7
Gross unrealized losses, less than twelve months 0.0 (0.4)
Over 12 months, Fair Market Value 91.1 1,287.2
Over 12 months, Gross Unrealized Losses (2.4) (101.1)
Total, Fair Market Value 96.9 1,333.9
Total, Gross Unrealized Losses $ (2.4) $ (101.5)
Number of Securities | security 12 558
Non-U.S. government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 118.8 $ 0.2
Gross unrealized losses, less than twelve months (0.9) 0.0
Over 12 months, Fair Market Value 43.7 95.5
Over 12 months, Gross Unrealized Losses (1.5) (5.9)
Total, Fair Market Value 162.5 95.7
Total, Gross Unrealized Losses $ (2.4) $ (5.9)
Number of Securities | security 41 12
Non-agency commercial mortgage-backed securities    
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  
Agency residential mortgage-backed securities    
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  
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   5.8
Over 12 months, Gross Unrealized Losses   (0.8)
Total, Fair Market Value   5.8
Total, Gross Unrealized Losses   $ (0.8)
Number of Securities | security   1
Asset-backed    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 17.2 $ 32.9
Gross unrealized losses, less than twelve months (0.1) (0.2)
Over 12 months, Fair Market Value 0.0 180.1
Over 12 months, Gross Unrealized Losses 0.0 (6.5)
Total, Fair Market Value 17.2 213.0
Total, Gross Unrealized Losses $ (0.1) $ (6.7)
Number of Securities | security 14 53