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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details)
$ in Millions
Dec. 31, 2021
USD ($)
security
Security
Dec. 31, 2020
USD ($)
security
Security
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 2,014.6 $ 147.8
Gross unrealized losses, less than twelve months (30.9) (0.8)
Over 12 months, Fair Market Value 72.0 2.1
Over 12 months, Gross Unrealized Losses (3.6) (0.2)
Total, Fair Market Value 2,086.6 149.9
Total, Gross Unrealized Losses $ (34.5) $ (1.0)
Number of Securities | security 644 64
Fixed income securities — Available for sale    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 2,010.2 $ 139.1
Gross unrealized losses, less than twelve months (30.9) (0.8)
Over 12 months, Fair Market Value 72.0 2.1
Over 12 months, Gross Unrealized Losses (3.6) 0.0
Total, Fair Market Value 2,082.2 141.2
Total, Gross Unrealized Losses $ (34.5) $ (0.8)
Number of Securities | Security 637 49
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 4.4 $ 8.7
Gross unrealized losses, less than twelve months 0.0 0.0
Over 12 months, Fair Market Value 0.0 0.0
Over 12 months, Gross Unrealized Losses 0.0 (0.2)
Total, Fair Market Value 4.4 8.7
Total, Gross Unrealized Losses $ 0.0 $ (0.2)
Number of Securities 7 15
U.S. government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 309.1 $ 58.7
Gross unrealized losses, less than twelve months (4.0) (0.6)
Over 12 months, Fair Market Value 29.3 0.0
Over 12 months, Gross Unrealized Losses (1.9) 0.0
Total, Fair Market Value 338.4 58.7
Total, Gross Unrealized Losses $ (5.9) $ (0.6)
Number of Securities | security 51 10
U.S. agency    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 6.1  
Gross unrealized losses, less than twelve months 0.0  
Over 12 months, Fair Market Value 0.0  
Over 12 months, Gross Unrealized Losses 0.0  
Total, Fair Market Value 6.1  
Total, Gross Unrealized Losses $ 0.0  
Number of Securities | security 5  
Municipal    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 29.7  
Gross unrealized losses, less than twelve months (0.5)  
Over 12 months, Fair Market Value 0.0  
Over 12 months, Gross Unrealized Losses 0.0  
Total, Fair Market Value 29.7  
Total, Gross Unrealized Losses $ (0.5)  
Number of Securities | security 20  
Corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 804.6 $ 39.6
Gross unrealized losses, less than twelve months (11.7) (0.1)
Over 12 months, Fair Market Value 23.6 0.0
Over 12 months, Gross Unrealized Losses (1.3) 0.0
Total, Fair Market Value 828.2 39.6
Total, Gross Unrealized Losses $ (13.0) $ (0.1)
Number of Securities | security 400 16
Non-U.S. government-backed corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 114.4  
Gross unrealized losses, less than twelve months (1.9)  
Over 12 months, Fair Market Value 0.0  
Over 12 months, Gross Unrealized Losses 0.0  
Total, Fair Market Value 114.4  
Total, Gross Unrealized Losses $ (1.9)  
Number of Securities | security 10  
Non-U.S government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 181.8 $ 7.5
Gross unrealized losses, less than twelve months (1.2) 0.0
Over 12 months, Fair Market Value 3.1 0.0
Over 12 months, Gross Unrealized Losses 0.0 0.0
Total, Fair Market Value 184.9 7.5
Total, Gross Unrealized Losses $ (1.2) $ 0.0
Number of Securities | security 35 4
Agency mortgage-backed    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 564.5 $ 33.3
Gross unrealized losses, less than twelve months (11.6) (0.1)
Over 12 months, Fair Market Value 16.0 2.1
Over 12 months, Gross Unrealized Losses (0.4) 0.0
Total, Fair Market Value 580.5 35.4
Total, Gross Unrealized Losses $ (12.0) $ (0.1)
Number of Securities | security 116 19