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Investments Available for Sale Securities (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Jun. 30, 2024
Dec. 31, 2023
Sep. 30, 2023
Jun. 30, 2023
Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost $ 2,124.6   $ 2,048.0      
Debt securities, available-for-sale, gross unrealized gain 27.0   14.9      
Debt securities, available-for-sale, gross unrealized loss (84.4)   (123.9)      
Fixed maturity securities, fair value 2,065.8   1,936.3      
Total AFS, amortized cost 2,155.2   2,081.1      
Total AFS, gross unrealized gain 27.0   14.9      
Total AFS, gross unrealized loss (84.4)   (123.9)      
Short-term investments at fair value (amortized cost $30.6 at September 30, 2024 and $33.1 at December 31, 2023) 30.6   33.1      
Total AFS investments 2,096.4   1,969.4      
Debt Securities, Available-for-sale, Allowance for Credit Loss (1.4) $ (1.9) (2.7) $ (3.2) $ (4.3) $ (4.5)
US Treasury Securities [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 62.4   60.3      
Debt securities, available-for-sale, gross unrealized gain 1.3   0.6      
Debt securities, available-for-sale, gross unrealized loss (1.6)   (2.5)      
Fixed maturity securities, fair value 62.1   58.4      
Debt Securities, Available-for-sale, Allowance for Credit Loss 0.0   0.0      
US Government Agencies Debt Securities [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost     2.2      
Debt securities, available-for-sale, gross unrealized gain     0.0      
Debt securities, available-for-sale, gross unrealized loss     (0.1)      
Fixed maturity securities, fair value     2.1      
Debt Securities, Available-for-sale, Allowance for Credit Loss     0.0      
US States and Political Subdivisions Debt Securities [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 181.2   212.3      
Debt securities, available-for-sale, gross unrealized gain 5.0   3.1      
Debt securities, available-for-sale, gross unrealized loss (3.2)   (5.2)      
Fixed maturity securities, fair value 183.0   210.2      
Debt Securities, Available-for-sale, Allowance for Credit Loss 0.0   0.0      
Corporate Debt Securities [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 930.9   952.8      
Debt securities, available-for-sale, gross unrealized gain 14.8   8.3      
Debt securities, available-for-sale, gross unrealized loss (39.4)   (63.2)      
Fixed maturity securities, fair value 905.7   895.8      
Debt Securities, Available-for-sale, Allowance for Credit Loss (0.6)   (2.1)      
Residential Mortgage Backed Securities [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 487.9   399.3      
Debt securities, available-for-sale, gross unrealized gain 2.0   0.9      
Debt securities, available-for-sale, gross unrealized loss (30.5)   (38.0)      
Fixed maturity securities, fair value 459.4   362.2      
Debt Securities, Available-for-sale, Allowance for Credit Loss 0.0   0.0      
Commercial Mortgage Backed Securities [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 64.9   70.2      
Debt securities, available-for-sale, gross unrealized gain 0.0   0.0      
Debt securities, available-for-sale, gross unrealized loss (3.9)   (6.4)      
Fixed maturity securities, fair value 60.7   63.8      
Debt Securities, Available-for-sale, Allowance for Credit Loss (0.3)   0.0      
Asset-backed Securities [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 209.5   131.8      
Debt securities, available-for-sale, gross unrealized gain 3.3   1.0      
Debt securities, available-for-sale, gross unrealized loss (2.7)   (4.8)      
Fixed maturity securities, fair value 210.1   128.0      
Debt Securities, Available-for-sale, Allowance for Credit Loss 0.0   0.0      
Foreign Government Debt            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 12.7   12.7      
Debt securities, available-for-sale, gross unrealized gain 0.0   0.0      
Debt securities, available-for-sale, gross unrealized loss (2.4)   (2.3)      
Fixed maturity securities, fair value 10.3   10.4      
Debt Securities, Available-for-sale, Allowance for Credit Loss 0.0   0.0      
Collateralized Loan Obligations [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 53.3   92.2      
Debt securities, available-for-sale, gross unrealized gain 0.0   0.0      
Debt securities, available-for-sale, gross unrealized loss (0.2)   (0.7)      
Fixed maturity securities, fair value 53.1   91.5      
Debt Securities, Available-for-sale, Allowance for Credit Loss 0.0   0.0      
Other Debt Obligations [Member]            
Debt Securities, Available-for-sale [Line Items]            
Debt securities, available-for-sale, amortized cost 121.8   114.2      
Debt securities, available-for-sale, gross unrealized gain 0.6   1.0      
Debt securities, available-for-sale, gross unrealized loss (0.5)   (0.7)      
Fixed maturity securities, fair value 121.4   113.9      
Debt Securities, Available-for-sale, Allowance for Credit Loss (0.5)   (0.6)      
Short-term Investments [Member]            
Debt Securities, Available-for-sale [Line Items]            
Total AFS, amortized cost 30.6   33.1      
Total AFS, gross unrealized gain 0.0   0.0      
Total AFS, gross unrealized loss 0.0   0.0      
Debt Securities, Available-for-sale, Allowance for Credit Loss $ 0.0   $ 0.0