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Investments - Continuous Loss Position (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value $ 200.6 $ 341.9
Less than 12 months, Gross Unrealized Losses 0.0 (3.3)
12 months or more, Fair Value 81.8 32.8
12 months or more, Gross Unrealized Losses (4.1) (3.9)
Total, Fair Value 282.4 374.7
Total, Gross Unrealized Losses (4.1) (7.2)
U.S. government and agencies    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 4.3 17.9
Less than 12 months, Gross Unrealized Losses 0.0 (0.4)
12 months or more, Fair Value 10.5 1.1
12 months or more, Gross Unrealized Losses (0.2) (0.1)
Total, Fair Value 14.8 19.0
Total, Gross Unrealized Losses (0.2) (0.5)
States and other territories    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 1.5 3.6
Less than 12 months, Gross Unrealized Losses 0.0 (0.1)
12 months or more, Fair Value 7.4 4.6
12 months or more, Gross Unrealized Losses (0.4) (0.5)
Total, Fair Value 8.9 8.2
Total, Gross Unrealized Losses (0.4) (0.6)
Corporate securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 5.7 30.5
Less than 12 months, Gross Unrealized Losses 0.0 (1.5)
12 months or more, Fair Value 37.4 11.1
12 months or more, Gross Unrealized Losses (1.3) (0.9)
Total, Fair Value 43.1 41.6
Total, Gross Unrealized Losses (1.3) (2.4)
Foreign securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 0.0 0.0
Less than 12 months, Gross Unrealized Losses 0.0 0.0
12 months or more, Fair Value 0.9 0.8
12 months or more, Gross Unrealized Losses 0.0 (0.1)
Total, Fair Value 0.9 0.8
Total, Gross Unrealized Losses 0.0 (0.1)
Residential mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 0.0 6.3
Less than 12 months, Gross Unrealized Losses 0.0 (0.3)
12 months or more, Fair Value 11.6 7.6
12 months or more, Gross Unrealized Losses (1.3) (1.3)
Total, Fair Value 11.6 13.9
Total, Gross Unrealized Losses (1.3) (1.6)
Commercial mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 0.4 1.9
Less than 12 months, Gross Unrealized Losses 0.0 0.0
12 months or more, Fair Value 5.8 3.9
12 months or more, Gross Unrealized Losses (0.6) (0.7)
Total, Fair Value 6.2 5.8
Total, Gross Unrealized Losses (0.6) (0.7)
Asset backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 1.6 5.5
Less than 12 months, Gross Unrealized Losses 0.0 (0.2)
12 months or more, Fair Value 8.2 3.7
12 months or more, Gross Unrealized Losses (0.3) (0.3)
Total, Fair Value 9.8 9.2
Total, Gross Unrealized Losses (0.3) (0.5)
U.S. government and agencies    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 137.7 129.1
Less than 12 months, Gross Unrealized Losses 0.0 (0.2)
12 months or more, Fair Value 0.0 0.0
12 months or more, Gross Unrealized Losses 0.0 0.0
Total, Fair Value 137.7 129.1
Total, Gross Unrealized Losses 0.0 (0.2)
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 34.5 147.1
Less than 12 months, Gross Unrealized Losses 0.0 (0.6)
12 months or more, Fair Value 0.0 0.0
12 months or more, Gross Unrealized Losses 0.0 0.0
Total, Fair Value 34.5 147.1
Total, Gross Unrealized Losses 0.0 $ (0.6)
Corporate securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 14.9  
Less than 12 months, Gross Unrealized Losses 0.0  
12 months or more, Fair Value 0.0  
12 months or more, Gross Unrealized Losses 0.0  
Total, Fair Value 14.9  
Total, Gross Unrealized Losses $ 0.0