Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 28.0	$ 39.8
Included in total net realized and unrealized investment gains (losses)		$ (5.3)
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)-net appreciation (depreciation) on available-for-sale securities	0.3	$ (1.1)
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
Settlements	(0.5)	$ (1.3)
Purchases and Sales:
Ending Balance	27.8	32.1
Change in unrealized gain (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	$ 0.3	$ (1.1)
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)-net appreciation (depreciation) on available-for-sale securities	0.1	(0.6)
Settlements	(0.3)	(1.1)
Purchases and Sales:
Ending Balance	9.1	12.3
Change in unrealized gain (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	0.1	(0.6)
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.1	0.1
Purchases and Sales:
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)-net appreciation (depreciation) on available-for-sale securities	0.2	(0.5)
Settlements	(0.2)	(0.2)
Purchases and Sales:
Ending Balance	7.5	10.6
Change in unrealized gain (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	0.2	(0.5)
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)-net appreciation (depreciation) on available-for-sale securities	0.3	(1.1)
Settlements	(0.5)	(1.3)
Purchases and Sales:
Ending Balance	16.7	23.0
Change in unrealized gain (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	0.3	(1.1)
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 total net realized and unrealized investment gains (losses)		(5.3)
Purchases and Sales:
Ending Balance	$ 11.1	$ 9.1