v3.26.3
Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Beginning Balance $ 35.1 $ 38.1 $ 36.2 $ 38.2
Total gains (losses) Included in net realized and unrealized investment gains (losses) $ (6.4) $ 0.4 $ (6.3) $ 1.2
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Gain Loss On Investments From Continuing Operations Gain Loss On Investments From Continuing Operations Gain Loss On Investments From Continuing Operations Gain Loss On Investments From Continuing Operations
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities $ (0.2) $ 0.2 $ (0.2) $ 0.6
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration] OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax
Purchase     $ 0.3  
Issuance     0.1  
Settlements $ (0.5) $ (0.3) (2.1) $ (1.6)
Ending Balance 28.0 38.4 28.0 38.4
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period $ (0.2) $ 0.2 $ (0.2) $ 0.6
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Gain Loss On Investments From Continuing Operations Gain Loss On Investments From Continuing Operations Gain Loss On Investments From Continuing Operations Gain Loss On Investments From Continuing Operations
Municipals [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Beginning Balance $ 6.7 $ 7.5 $ 7.4 $ 7.6
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities (0.1) 0.1 (0.1) 0.3
Settlements (0.2) (0.1) (0.9) (0.4)
Ending Balance 6.4 7.5 6.4 7.5
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period (0.1) 0.1 (0.1) 0.3
Commercial Mortgage-Backed [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Beginning Balance 4.7 5.1 4.9 5.9
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities (0.1) 0.1 (0.1) 0.3
Settlements (0.1) (0.2) (0.3) (1.2)
Ending Balance 4.5 5.0 4.5 5.0
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period (0.1) 0.1 (0.1) 0.3
Debt Securities [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Beginning Balance 11.4 12.6 12.3 13.5
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities (0.2) 0.2 (0.2) 0.6
Settlements (0.3) (0.3) (1.2) (1.6)
Ending Balance 10.9 12.5 10.9 12.5
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period (0.2) 0.2 (0.2) 0.6
Equities and Other [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Beginning Balance 23.7 25.5 23.9 24.7
Total gains (losses) Included in net realized and unrealized investment gains (losses) (6.4) 0.4 (6.3) 1.2
Purchase     0.3  
Issuance     0.1  
Settlements (0.2)   (0.9)  
Ending Balance $ 17.1 $ 25.9 $ 17.1 $ 25.9