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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 27.2	$ 25.5	$ 29.4	$ 31.0
Transfers out of Level 3				(3.5)
Included in total net realized and unrealized investment gains	4.5	0.2	4.6	0.4
Included in other comprehensive income (loss)-net appreciation (depreciation) on available-for-sale securities		0.3	(0.4)	0.2
Purchases and Sales:
Sales	(0.6)	(0.3)	(2.5)	(2.4)
Ending Balance	31.1	25.7	31.1	25.7
Change in unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period		0.3	(0.4)	0.3
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	5.5	7.4	7.0	12.1
Transfers out of Level 3				(3.5)
Included in other comprehensive income (loss)-net appreciation (depreciation) on available-for-sale securities				0.1
Purchases and Sales:
Sales	(0.1)	(0.1)	(1.6)	(1.4)
Ending Balance	5.4	7.3	5.4	7.3
Change in unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period				0.2
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.4	0.5	0.5	0.6
Purchases and Sales:
Sales	(0.3)		(0.4)	(0.1)
Ending Balance	0.1	0.5	0.1	0.5
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	11.7	11.8	12.4	12.7
Included in other comprehensive income (loss)-net appreciation (depreciation) on available-for-sale securities		0.3	(0.4)	0.1
Purchases and Sales:
Sales	(0.2)	(0.2)	(0.5)	(0.9)
Ending Balance	11.5	11.9	11.5	11.9
Change in unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period		0.3	(0.4)	0.1
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	17.6	19.7	19.9	25.4
Transfers out of Level 3				(3.5)
Included in other comprehensive income (loss)-net appreciation (depreciation) on available-for-sale securities		0.3	(0.4)	0.2
Purchases and Sales:
Sales	(0.6)	(0.3)	(2.5)	(2.4)
Ending Balance	17.0	19.7	17.0	19.7
Change in unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period		0.3	(0.4)	0.3
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	9.6	5.8	9.5	5.6
Included in total net realized and unrealized investment gains	4.5	0.2	4.6	0.4
Purchases and Sales:
Ending Balance	$ 14.1	$ 6.0	$ 14.1	$ 6.0