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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details)
$ in Millions
Dec. 31, 2023
USD ($)
security
Dec. 31, 2022
USD ($)
security
Security
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 196.5 $ 2,321.4
Gross unrealized losses, less than twelve months (2.1) (145.4)
Over 12 months, Fair Market Value 2,802.0 1,323.7
Over 12 months, Gross Unrealized Losses (219.7) (193.6)
Total, Fair Market Value 2,998.5 3,645.1
Total, Gross Unrealized Losses $ (221.8) $ (339.0)
Number of Securities | security 950 1,166
Fixed income securities — Available for sale    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value   $ 2,269.5
Gross unrealized losses, less than twelve months   (145.0)
Over 12 months, Fair Market Value   1,323.7
Over 12 months, Gross Unrealized Losses   (193.6)
Total, Fair Market Value   3,593.2
Total, Gross Unrealized Losses   $ (338.6)
Number of Securities | Security   1,165
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value   $ 51.9
Gross unrealized losses, less than twelve months   (0.4)
Over 12 months, Fair Market Value   0.0
Over 12 months, Gross Unrealized Losses   0.0
Total, Fair Market Value   51.9
Total, Gross Unrealized Losses   $ (0.4)
Number of Securities | Security   1
U.S. government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 105.5 $ 741.1
Gross unrealized losses, less than twelve months (0.5) (30.6)
Over 12 months, Fair Market Value 673.3 203.4
Over 12 months, Gross Unrealized Losses (26.2) (20.1)
Total, Fair Market Value 778.8 944.5
Total, Gross Unrealized Losses $ (26.7) $ (50.7)
Number of Securities | security 74 118
U.S. agency    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 0.0 $ 7.1
Gross unrealized losses, less than twelve months 0.0 (0.4)
Over 12 months, Fair Market Value 7.2 1.7
Over 12 months, Gross Unrealized Losses (0.3) 0.0
Total, Fair Market Value 7.2 8.8
Total, Gross Unrealized Losses $ (0.3) $ (0.4)
Number of Securities | security 1 5
Municipal    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 11.1 $ 133.1
Gross unrealized losses, less than twelve months (1.0) (7.1)
Over 12 months, Fair Market Value 117.0 16.4
Over 12 months, Gross Unrealized Losses (4.0) (2.3)
Total, Fair Market Value 128.1 149.5
Total, Gross Unrealized Losses $ (5.0) $ (9.4)
Number of Securities | security 54 59
Corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 46.7 $ 1,104.7
Gross unrealized losses, less than twelve months (0.4) (77.6)
Over 12 months, Fair Market Value 1,287.2 568.2
Over 12 months, Gross Unrealized Losses (101.1) (91.5)
Total, Fair Market Value 1,333.9 1,672.9
Total, Gross Unrealized Losses $ (101.5) $ (169.1)
Number of Securities | security 558 701
Non-U.S. government-backed corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 0.2 $ 11.3
Gross unrealized losses, less than twelve months 0.0 (0.5)
Over 12 months, Fair Market Value 95.5 99.0
Over 12 months, Gross Unrealized Losses (5.9) (8.3)
Total, Fair Market Value 95.7 110.3
Total, Gross Unrealized Losses $ (5.9) $ (8.8)
Number of Securities | security 12 16
Non-U.S government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 32.9 $ 63.8
Gross unrealized losses, less than twelve months (0.2) (2.9)
Over 12 months, Fair Market Value 180.1 135.9
Over 12 months, Gross Unrealized Losses (6.5) (8.6)
Total, Fair Market Value 213.0 199.7
Total, Gross Unrealized Losses $ (6.7) $ (11.5)
Number of Securities | security 53 45
Non-agency commercial mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 0.0 $ 5.6
Gross unrealized losses, less than twelve months 0.0 (1.0)
Over 12 months, Fair Market Value 5.8 0.0
Over 12 months, Gross Unrealized Losses (0.8) 0.0
Total, Fair Market Value 5.8 5.6
Total, Gross Unrealized Losses $ (0.8) $ (1.0)
Number of Securities | security 1 1
Agency residential mortgage-backed securities    
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  
Collateralized Mortgage-Backed Securities    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value   $ 202.8
Gross unrealized losses, less than twelve months   (24.9)
Over 12 months, Fair Market Value   299.1
Over 12 months, Gross Unrealized Losses   (62.8)
Total, Fair Market Value   501.9
Total, Gross Unrealized Losses   $ (87.7)
Number of Securities | security   220