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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details)
$ in Millions
Jun. 30, 2023
USD ($)
security
Security
Dec. 31, 2022
USD ($)
security
Security
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 1,175.6 $ 2,321.4
Gross unrealized losses, less than twelve months (32.5) (145.4)
Over 12 months, Fair Market Value 2,582.7 1,323.7
Over 12 months, Gross Unrealized Losses (288.9) (193.6)
Total, Fair Market Value 3,758.3 3,645.1
Total, Gross Unrealized Losses $ (321.4) $ (339.0)
Number of Securities | security 1,193 1,166
Fixed income securities — Available for sale    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 1,163.8 $ 2,269.5
Gross unrealized losses, less than twelve months (32.5) (145.0)
Over 12 months, Fair Market Value 2,582.7 1,323.7
Over 12 months, Gross Unrealized Losses (288.9) (193.6)
Total, Fair Market Value 3,746.5 3,593.2
Total, Gross Unrealized Losses $ (321.4) $ (338.6)
Number of Securities | Security 1,188 1,165
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 11.8 $ 51.9
Gross unrealized losses, less than twelve months 0.0 (0.4)
Over 12 months, Fair Market Value 0.0 0.0
Over 12 months, Gross Unrealized Losses 0.0 0.0
Total, Fair Market Value 11.8 51.9
Total, Gross Unrealized Losses $ 0.0 $ (0.4)
Number of Securities 5 1
U.S. government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 494.8 $ 741.1
Gross unrealized losses, less than twelve months (12.8) (30.6)
Over 12 months, Fair Market Value 519.1 203.4
Over 12 months, Gross Unrealized Losses (38.3) (20.1)
Total, Fair Market Value 1,013.9 944.5
Total, Gross Unrealized Losses $ (51.1) $ (50.7)
Number of Securities | security 105 118
U.S. agency    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 7.0 $ 7.1
Gross unrealized losses, less than twelve months (0.4) (0.4)
Over 12 months, Fair Market Value 1.1 1.7
Over 12 months, Gross Unrealized Losses 0.0 0.0
Total, Fair Market Value 8.1 8.8
Total, Gross Unrealized Losses $ (0.4) $ (0.4)
Number of Securities | security 4 5
Municipal    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 90.9 $ 133.1
Gross unrealized losses, less than twelve months (3.7) (7.1)
Over 12 months, Fair Market Value 58.7 16.4
Over 12 months, Gross Unrealized Losses (5.1) (2.3)
Total, Fair Market Value 149.6 149.5
Total, Gross Unrealized Losses $ (8.8) $ (9.4)
Number of Securities | security 60 59
Corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 467.4 $ 1,104.7
Gross unrealized losses, less than twelve months (13.1) (77.6)
Over 12 months, Fair Market Value 1,254.7 568.2
Over 12 months, Gross Unrealized Losses (142.1) (91.5)
Total, Fair Market Value 1,722.1 1,672.9
Total, Gross Unrealized Losses $ (155.2) $ (169.1)
Number of Securities | security 727 701
Non-U.S. government-backed corporate    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 5.2 $ 11.3
Gross unrealized losses, less than twelve months 0.0 (0.5)
Over 12 months, Fair Market Value 105.7 99.0
Over 12 months, Gross Unrealized Losses (8.7) (8.3)
Total, Fair Market Value 110.9 110.3
Total, Gross Unrealized Losses $ (8.7) $ (8.8)
Number of Securities | security 16 16
Non-U.S government    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 80.1 $ 63.8
Gross unrealized losses, less than twelve months (1.2) (2.9)
Over 12 months, Fair Market Value 182.2 135.9
Over 12 months, Gross Unrealized Losses (10.8) (8.6)
Total, Fair Market Value 262.3 199.7
Total, Gross Unrealized Losses $ (12.0) $ (11.5)
Number of Securities | security 61 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 5.6 0.0
Over 12 months, Gross Unrealized Losses (0.9) 0.0
Total, Fair Market Value 5.6 5.6
Total, Gross Unrealized Losses $ (0.9) $ (1.0)
Number of Securities | security 1 1
Agency mortgage-backed    
Debt Securities, Available-for-sale [Line Items]    
0-12 months, Fair Market Value $ 18.4 $ 202.8
Gross unrealized losses, less than twelve months (1.3) (24.9)
Over 12 months, Fair Market Value 455.6 299.1
Over 12 months, Gross Unrealized Losses (83.0) (62.8)
Total, Fair Market Value 474.0 501.9
Total, Gross Unrealized Losses $ (84.3) $ (87.7)
Number of Securities | security 214 220