Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 32.1	$ 28.0	$ 39.8	$ 29.4
Included in total net realized and unrealized investment gains	5.3			0.1
Included in other comprehensive loss-net appreciation depreciation on available-for-sale securities	(0.5)		(1.6)	(0.4)
Purchases and Sales:
Sales	(3.3)	(0.8)	(4.6)	(1.9)
Ending Balance	33.6	27.2	33.6	27.2
Change in unrealized losses for the period included in other comprehensive loss for assets held at the end of the period	(0.5)		(1.6)	(0.4)
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	12.3	6.2	14.0	7.0
Included in other comprehensive loss-net appreciation depreciation on available-for-sale securities	(0.2)		(0.8)
Purchases and Sales:
Sales	(1.1)	(0.7)	(2.2)	(1.5)
Ending Balance	11.0	5.5	11.0	5.5
Change in unrealized losses for the period included in other comprehensive loss for assets held at the end of the period	(0.2)		(0.8)
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.1	0.4	0.1	0.5
Purchases and Sales:
Sales				(0.1)
Ending Balance	0.1	0.4	0.1	0.4
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	10.6	11.8	11.3	12.4
Included in other comprehensive loss-net appreciation depreciation on available-for-sale securities	(0.3)		(0.8)	(0.4)
Purchases and Sales:
Sales	(2.2)	(0.1)	(2.4)	(0.3)
Ending Balance	8.1	11.7	8.1	11.7
Change in unrealized losses for the period included in other comprehensive loss for assets held at the end of the period	(0.3)		(0.8)	(0.4)
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	23.0	18.4	25.4	19.9
Included in other comprehensive loss-net appreciation depreciation on available-for-sale securities	(0.5)		(1.6)	(0.4)
Purchases and Sales:
Sales	(3.3)	(0.8)	(4.6)	(1.9)
Ending Balance	19.2	17.6	19.2	17.6
Change in unrealized losses for the period included in other comprehensive loss for assets held at the end of the period	(0.5)		(1.6)	(0.4)
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	9.1	9.6	14.4	9.5
Included in total net realized and unrealized investment gains	5.3			0.1
Purchases and Sales:
Ending Balance	$ 14.4	$ 9.6	$ 14.4	$ 9.6