Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 39.8	$ 29.4
Included in total net realized and unrealized investment gains (losses)	$ (3.3)	$ 4.6
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
Included in other comprehensive (loss) - net appreciation (depreciation) on available-for-sale securities	$ (2.2)	$ (0.3)
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
Purchases and Sales:
Purchases		$ 10.0
Sales	$ (6.3)	(3.9)
Ending Balance	28.0	39.8
Changes in unrealized losses for the period included in other comprehensive income (loss) for assets held at the end of the year	$ (2.2)	$ (0.5)
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	$ 14.0	$ 7.0
Included in other comprehensive (loss) - net appreciation (depreciation) on available-for-sale securities	(1.1)	(0.1)
Purchases and Sales:
Purchases		10.0
Sales	(3.6)	(2.9)
Ending Balance	9.3	14.0
Changes in unrealized losses for the period included in other comprehensive income (loss) for assets held at the end of the year	(1.1)
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.1	0.5
Purchases and Sales:
Sales		(0.4)
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	11.3	12.4
Included in other comprehensive (loss) - net appreciation (depreciation) on available-for-sale securities	(1.1)	(0.5)
Purchases and Sales:
Sales	(2.7)	(0.6)
Ending Balance	7.5	11.3
Changes in unrealized losses for the period included in other comprehensive income (loss) for assets held at the end of the year	(1.1)	(0.5)
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	25.4	19.9
Included in other comprehensive (loss) - net appreciation (depreciation) on available-for-sale securities	(2.2)	(0.6)
Purchases and Sales:
Purchases		10.0
Sales	(6.3)	(3.9)
Ending Balance	16.9	25.4
Changes in unrealized losses for the period included in other comprehensive income (loss) for assets held at the end of the year	(2.2)	(0.5)
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	14.4	9.5
Included in total net realized and unrealized investment gains (losses)	(3.3)	4.6
Included in other comprehensive (loss) - net appreciation (depreciation) on available-for-sale securities		0.3
Purchases and Sales:
Ending Balance	$ 11.1	$ 14.4