Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 38.2	$ 27.1
Transfers out of Level 3		(0.1)
Included in net realized and unrealized investment gains (losses)	$ (0.1)
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
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	$ 0.3	$ (0.2)
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
Settlements	$ (1.1)	$ (0.4)
Ending Balance	37.3	26.4
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	$ 0.3	$ (0.2)
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
Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 7.6	$ 8.6
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	0.2	(0.1)
Settlements	(0.2)	(0.3)
Ending Balance	7.6	8.2
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	(0.1)
Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		0.1
Transfers out of Level 3		(0.1)
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	5.9	7.0
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	0.1	(0.1)
Settlements	(0.9)	(0.1)
Ending Balance	5.1	6.8
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)
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	13.5	15.7
Transfers out of Level 3		(0.1)
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	0.3	(0.2)
Settlements	(1.1)	(0.4)
Ending Balance	12.7	15.0
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	0.3	(0.2)
Equities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	24.7	11.4
Included in net realized and unrealized investment gains (losses)	(0.1)
Ending Balance	$ 24.6	$ 11.4