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Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details)
$ in Millions
Sep. 30, 2024
USD ($)
security
Dec. 31, 2023
USD ($)
security
Dec. 31, 2022
USD ($)
security
Security
Debt Securities, Available-for-sale [Line Items]      
0-12 months, Fair Market Value $ 244.7 $ 196.5 $ 2,321.4
Gross unrealized losses, less than twelve months (1.2) (2.1) (145.4)
Over 12 months, Fair Market Value 2,169.1 2,802.0 1,323.7
Over 12 months, Gross Unrealized Losses (142.3) (219.7) (193.6)
Total, Fair Market Value $ 2,413.8 2,998.5 3,645.1
Total, Gross Unrealized Losses   $ (221.8) $ (339.0)
Number of Securities | security 781 950 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 $ 126.0 $ 105.5 $ 741.1
Gross unrealized losses, less than twelve months (0.7) (0.5) (30.6)
Over 12 months, Fair Market Value 560.2 673.3 203.4
Over 12 months, Gross Unrealized Losses (13.1) (26.2) (20.1)
Total, Fair Market Value $ 686.2 778.8 944.5
Total, Gross Unrealized Losses   $ (26.7) $ (50.7)
Number of Securities | security 65 74 118
U.S. agency      
Debt Securities, Available-for-sale [Line Items]      
0-12 months, Fair Market Value $ 0.0 $ 0.0 $ 7.1
Gross unrealized losses, less than twelve months 0.0 0.0 (0.4)
Over 12 months, Fair Market Value 7.4 7.2 1.7
Over 12 months, Gross Unrealized Losses (0.1) (0.3) 0.0
Total, Fair Market Value $ 7.4 7.2 8.8
Total, Gross Unrealized Losses   $ (0.3) $ (0.4)
Number of Securities | security 1 1 5
Municipal      
Debt Securities, Available-for-sale [Line Items]      
0-12 months, Fair Market Value $ 0.0 $ 11.1 $ 133.1
Gross unrealized losses, less than twelve months 0.0 (1.0) (7.1)
Over 12 months, Fair Market Value 66.2 117.0 16.4
Over 12 months, Gross Unrealized Losses (1.6) (4.0) (2.3)
Total, Fair Market Value $ 66.2 128.1 149.5
Total, Gross Unrealized Losses   $ (5.0) $ (9.4)
Number of Securities | security 37 54 59
Corporate      
Debt Securities, Available-for-sale [Line Items]      
0-12 months, Fair Market Value $ 59.0 $ 46.7 $ 1,104.7
Gross unrealized losses, less than twelve months (0.3) (0.4) (77.6)
Over 12 months, Fair Market Value 920.7 1,287.2 568.2
Over 12 months, Gross Unrealized Losses (58.2) (101.1) (91.5)
Total, Fair Market Value $ 979.7 1,333.9 1,672.9
Total, Gross Unrealized Losses   $ (101.5) $ (169.1)
Number of Securities | security 435 558 701
Non-U.S. government-backed corporate      
Debt Securities, Available-for-sale [Line Items]      
0-12 months, Fair Market Value $ 4.2 $ 0.2 $ 11.3
Gross unrealized losses, less than twelve months 0.0 0.0 (0.5)
Over 12 months, Fair Market Value 102.9 95.5 99.0
Over 12 months, Gross Unrealized Losses (3.1) (5.9) (8.3)
Total, Fair Market Value $ 107.1 95.7 110.3
Total, Gross Unrealized Losses   $ (5.9) $ (8.8)
Number of Securities | security 12 12 16
Non-U.S government      
Debt Securities, Available-for-sale [Line Items]      
0-12 months, Fair Market Value $ 23.6 $ 32.9 $ 63.8
Gross unrealized losses, less than twelve months 0.0 (0.2) (2.9)
Over 12 months, Fair Market Value 114.6 180.1 135.9
Over 12 months, Gross Unrealized Losses (2.2) (6.5) (8.6)
Total, Fair Market Value $ 138.2 213.0 199.7
Total, Gross Unrealized Losses   $ (6.7) $ (11.5)
Number of Securities | security 37 53 45
Agency commercial mortgage-backed securities      
Debt Securities, Available-for-sale [Line Items]      
0-12 months, Fair Market Value $ 0.0 $ 0.0 $ 5.6
Gross unrealized losses, less than twelve months 0.0 0.0 (1.0)
Over 12 months, Fair Market Value 4.6 5.8 0.0
Over 12 months, Gross Unrealized Losses (0.5) (0.8) 0.0
Total, Fair Market Value $ 4.6 5.8 5.6
Total, Gross Unrealized Losses   $ (0.8) $ (1.0)
Number of Securities | security 1 1 1
Residential Mortgage-Backed Securities      
Debt Securities, Available-for-sale [Line Items]      
0-12 months, Fair Market Value $ 13.1 $ 0.1  
Gross unrealized losses, less than twelve months 0.0 0.0  
Over 12 months, Fair Market Value 392.5 435.9  
Over 12 months, Gross Unrealized Losses (63.5) (74.9)  
Total, Fair Market Value $ 405.6 436.0  
Total, Gross Unrealized Losses   $ (74.9)  
Number of Securities | security 181 197  
Agency mortgage-backed      
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