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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 60.8	$ 60.8	$ 60.5	$ 61.7
Transfers into Level 3				1.3
Transfers out of Level 3	(1.2)	(4.6)	(1.2)	(4.6)
Included in total net realized investment gains		0.1	(0.1)	0.2
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	(0.2)	0.2	2.8	(1.3)
Purchases		3.1	0.3	6.2
Sales	(1.6)	(0.9)	(4.5)	(4.8)
Ending Balance	57.8	58.7	57.8	58.7
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	55.9	55.7	55.6	56.7
Transfers into Level 3				1.3
Transfers out of Level 3	(1.2)	(4.6)	(1.2)	(4.6)
Included in total net realized investment gains		0.1	(0.1)	0.2
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	(0.3)	0.2	2.7	(1.4)
Purchases		3.1	0.3	6.2
Sales	(1.6)	(0.9)	(4.5)	(4.8)
Ending Balance	52.8	53.6	52.8	53.6
Fixed Maturities [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	34.4	27.0	34.4	25.7
Transfers out of Level 3	(1.2)		(1.2)
Included in total net realized investment gains			0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	(0.1)	0.4	1.5	(0.4)
Purchases		3.1		6.2
Sales	(0.8)	(0.5)	(2.5)	(1.5)
Ending Balance	32.3	30.0	32.3	30.0
Fixed Maturities [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	4.4	9.4	3.7	9.6
Transfers out of Level 3		(4.6)		(4.6)
Included in total net realized investment gains			(0.2)	0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	(0.1)		0.6	(0.2)
Purchases			0.3
Sales		(0.1)	(0.1)	(0.2)
Ending Balance	4.3	4.7	4.3	4.7
Fixed Maturities [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	16.7	18.0	17.0	21.4
Included in total net realized investment gains		0.1		0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	(0.1)	(0.1)	0.6	(0.7)
Sales	(0.4)	(0.3)	(1.4)	(3.1)
Ending Balance	16.2	17.7	16.2	17.7
Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.4	1.3	0.5
Transfers into Level 3				1.3
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities		(0.1)		(0.1)
Sales	(0.4)		(0.5)
Ending Balance		1.2		1.2
Equity Securities and Other Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	4.9	5.1	4.9	5.0
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.1		0.1	0.1
Ending Balance	$ 5.0	$ 5.1	$ 5.0	$ 5.1