Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 28.0	$ 39.8
Included in net realized and unrealized investment gains (losses)	0.3	$ (3.3)
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	$ (2.2)
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	$ (1.5)	$ (6.3)
Ending Balance	27.1	28.0
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the year	$ 0.3	$ (2.2)
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
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 9.3	$ 14.0
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	0.2	(1.1)
Settlements	(0.9)	(3.6)
Ending Balance	8.6	9.3
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the year	0.2	(1.1)
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.1	0.1
Ending Balance	0.1	0.1
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	7.5	11.3
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	0.1	(1.1)
Settlements	(0.6)	(2.7)
Ending Balance	7.0	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 year	0.1	(1.1)
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	16.9	25.4
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	0.3	(2.2)
Settlements	(1.5)	(6.3)
Ending Balance	15.7	16.9
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the year	0.3	(2.2)
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	11.1	14.4
Included in net realized and unrealized investment gains (losses)	0.3	(3.3)
Ending Balance	$ 11.4	$ 11.1