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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 51.3	$ 60.8	$ 55.8	$ 60.5
Transfers out of Level 3	(2.3)	(1.2)	(2.3)	(1.2)
Included in total net realized investment gains			0.3	(0.1)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	0.2	(0.2)	0.5	2.8
Purchases				0.3
Sales	(0.6)	(1.6)	(5.7)	(4.5)
Ending Balance	48.6	57.8	48.6	57.8
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	30.1	34.4	31.0	34.4
Transfers out of Level 3	(1.9)	(1.2)	(1.9)	(1.2)
Included in total net realized investment gains				0.1
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	0.2	(0.1)	0.6	1.5
Sales	(0.3)	(0.8)	(1.6)	(2.5)
Ending Balance	28.1	32.3	28.1	32.3
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	1.0	4.4	4.2	3.7
Included in total net realized investment gains			0.3	(0.2)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities		(0.1)	(0.2)	0.6
Purchases				0.3
Sales	(0.1)		(3.4)	(0.1)
Ending Balance	0.9	4.3	0.9	4.3
Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	14.6	16.7	15.0	17.0
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities		(0.1)	0.1	0.6
Sales	(0.2)	(0.4)	(0.7)	(1.4)
Ending Balance	14.4	16.2	14.4	16.2
Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		0.4		0.5
Sales		(0.4)		(0.5)
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	45.7	55.9	50.2	55.6
Transfers out of Level 3	(1.9)	(1.2)	(1.9)	(1.2)
Included in total net realized investment gains			0.3	(0.1)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	0.2	(0.3)	0.5	2.7
Purchases				0.3
Sales	(0.6)	(1.6)	(5.7)	(4.5)
Ending Balance	43.4	52.8	43.4	52.8
Equity Securities and Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	5.6	4.9	5.6	4.9
Transfers out of Level 3	(0.4)		(0.4)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities		0.1		0.1
Ending Balance	$ 5.2	$ 5.0	$ 5.2	$ 5.0