Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 33.6	$ 27.2	$ 39.8	$ 29.4
Included in total net realized and unrealized investment gains (losses)	(0.1)	4.5	(0.1)	4.6
Included in other comprehensive loss-net appreciation depreciation on available-for-sale securities	(0.7)		(2.3)	(0.4)
Purchases and Sales:
Sales	(0.8)	(0.6)	(5.4)	(2.5)
Ending Balance	32.0	31.1	32.0	31.1
Change in unrealized losses for the period included in other comprehensive loss for assets held at the end of the period	(0.7)		(2.3)	(0.4)
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	11.0	5.5	14.0	7.0
Included in other comprehensive loss-net appreciation depreciation on available-for-sale securities	(0.3)		(1.1)
Purchases and Sales:
Sales	(0.7)	(0.1)	(2.9)	(1.6)
Ending Balance	10.0	5.4	10.0	5.4
Change in unrealized losses for the period included in other comprehensive loss for assets held at the end of the period	(0.4)		(1.2)
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		(0.3)		(0.4)
Ending Balance	0.1	0.1	0.1	0.1
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	8.1	11.7	11.3	12.4
Included in other comprehensive loss-net appreciation depreciation on available-for-sale securities	(0.4)		(1.2)	(0.4)
Purchases and Sales:
Sales	(0.1)	(0.2)	(2.5)	(0.5)
Ending Balance	7.6	11.5	7.6	11.5
Change in unrealized losses for the period included in other comprehensive loss for assets held at the end of the period	(0.3)		(1.1)	(0.4)
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	19.2	17.6	25.4	19.9
Included in other comprehensive loss-net appreciation depreciation on available-for-sale securities	(0.7)		(2.3)	(0.4)
Purchases and Sales:
Sales	(0.8)	(0.6)	(5.4)	(2.5)
Ending Balance	17.7	17.0	17.7	17.0
Change in unrealized losses for the period included in other comprehensive loss for assets held at the end of the period	(0.7)		(2.3)	(0.4)
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	14.4	9.6	14.4	9.5
Included in total net realized and unrealized investment gains (losses)	(0.1)	4.5	(0.1)	4.6
Purchases and Sales:
Ending Balance	$ 14.3	$ 14.1	$ 14.3	$ 14.1