Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 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)	(5.3)	0.1
Included in other comprehensive income (loss)-net appreciation (depreciation) on available-for-sale securities	(1.1)	(0.4)
Purchases and Sales:
Sales	(1.3)	(1.1)
Ending Balance	32.1	28.0
Change in unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	(1.1)	(0.4)
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 income (loss)-net appreciation (depreciation) on available-for-sale securities	(0.6)
Purchases and Sales:
Sales	(1.1)	(0.8)
Ending Balance	12.3	6.2
Change in unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	(0.6)
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		(0.1)
Ending Balance	0.1	0.4
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 income (loss)-net appreciation (depreciation) on available-for-sale securities	(0.5)	(0.4)
Purchases and Sales:
Sales	(0.2)	(0.2)
Ending Balance	10.6	11.8
Change in unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	(0.5)	(0.4)
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	25.4	19.9
Included in other comprehensive income (loss)-net appreciation (depreciation) on available-for-sale securities	(1.1)	(0.4)
Purchases and Sales:
Sales	(1.3)	(1.1)
Ending Balance	23.0	18.4
Change in unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	(1.1)	(0.4)
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	14.4	9.5
Included in total net realized and unrealized investment gains (losses)	(5.3)	0.1
Purchases and Sales:
Ending Balance	$ 9.1	$ 9.6