Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 46.6	$ 55.8
Included in total net realized and unrealized investment gains	0.1
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.7)	0.2
Sales	(1.7)	(1.1)
Ending Balance	44.3	54.9
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	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	(0.5)	0.2
Sales	(1.1)	(0.4)
Ending Balance	25.3	30.8
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.9	4.2
Sales	(0.1)	(0.3)
Ending Balance	0.8	3.9
Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	14.2	15.0
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.2)
Sales	(0.5)	(0.4)
Ending Balance	13.5	14.6
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	41.9	50.2
Included in total net realized and unrealized investment gains	0.1
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.7)	0.2
Sales	(1.7)	(1.1)
Ending Balance	39.6	49.3
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
Ending Balance	$ 4.7	$ 5.6