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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 44.3	$ 54.9	$ 46.6	$ 55.8
Included in total net realized and unrealized investment gains		0.3	0.1	0.3
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.2)	0.1	(0.9)	0.3
Sales	(0.9)	(4.0)	(2.6)	(5.1)
Ending Balance	43.2	51.3	43.2	51.3
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	25.3	30.8	26.8	31.0
Included in total net realized and unrealized investment gains			0.1
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.1)	0.2	(0.6)	0.4
Sales	(0.8)	(0.9)	(1.9)	(1.3)
Ending Balance	24.4	30.1	24.4	30.1
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.8	3.9	0.9	4.2
Included in total net realized and unrealized investment gains		0.3		0.3
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities		(0.2)		(0.2)
Sales		(3.0)	(0.1)	(3.3)
Ending Balance	0.8	1.0	0.8	1.0
Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	13.5	14.6	14.2	15.0
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.1)	0.1	(0.3)	0.1
Sales	(0.1)	(0.1)	(0.6)	(0.5)
Ending Balance	13.3	14.6	13.3	14.6
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	39.6	49.3	41.9	50.2
Included in total net realized and unrealized investment gains		0.3	0.1	0.3
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.2)	0.1	(0.9)	0.3
Sales	(0.9)	(4.0)	(2.6)	(5.1)
Ending Balance	38.5	45.7	38.5	45.7
Equity Securities and Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	4.7	5.6	4.7	5.6
Ending Balance	$ 4.7	$ 5.6	$ 4.7	$ 5.6