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Investments - Continuous Loss Position (Details) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value $ 251.0 $ 341.9
Less than 12 months, Gross Unrealized Losses (1.1) (3.3)
12 months or more, Fair Value 84.0 32.8
12 months or more, Gross Unrealized Losses (6.1) (3.9)
Total, Fair Value 335.0 374.7
Total, Gross Unrealized Losses (7.2) (7.2)
U.S. government and agencies    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 10.2 17.9
Less than 12 months, Gross Unrealized Losses (0.1) (0.4)
12 months or more, Fair Value 11.9 1.1
12 months or more, Gross Unrealized Losses (0.3) (0.1)
Total, Fair Value 22.1 19.0
Total, Gross Unrealized Losses (0.4) (0.5)
States and other territories    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 1.4 3.6
Less than 12 months, Gross Unrealized Losses 0.0 (0.1)
12 months or more, Fair Value 8.0 4.6
12 months or more, Gross Unrealized Losses (0.6) (0.5)
Total, Fair Value 9.4 8.2
Total, Gross Unrealized Losses (0.6) (0.6)
Corporate securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 36.4 30.5
Less than 12 months, Gross Unrealized Losses (0.5) (1.5)
12 months or more, Fair Value 37.8 11.1
12 months or more, Gross Unrealized Losses (2.1) (0.9)
Total, Fair Value 74.2 41.6
Total, Gross Unrealized Losses (2.6) (2.4)
Foreign securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 0.0
Less than 12 months, Gross Unrealized Losses 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 7.6 6.3
Less than 12 months, Gross Unrealized Losses (0.3) (0.3)
12 months or more, Fair Value 11.4 7.6
12 months or more, Gross Unrealized Losses (2.0) (1.3)
Total, Fair Value 19.0 13.9
Total, Gross Unrealized Losses (2.3) (1.6)
Commercial mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 1.3 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 (0.7) (0.7)
Total, Fair Value 7.1 5.8
Total, Gross Unrealized Losses (0.7) (0.7)
Asset backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 5.6 5.5
Less than 12 months, Gross Unrealized Losses (0.1) (0.2)
12 months or more, Fair Value 8.2 3.7
12 months or more, Gross Unrealized Losses (0.4) (0.3)
Total, Fair Value 13.8 9.2
Total, Gross Unrealized Losses (0.5) (0.5)
U.S. government and agencies    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 123.4 129.1
Less than 12 months, Gross Unrealized Losses (0.1) (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 123.4 129.1
Total, Gross Unrealized Losses (0.1) (0.2)
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 47.5 147.1
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 47.5 147.1
Total, Gross Unrealized Losses 0.0 $ (0.6)
Corporate securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Fair Value 17.6  
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 17.6  
Total, Gross Unrealized Losses $ 0.0