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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 60.7	$ 63.5	$ 60.5	$ 61.7
Transfers into Level 3		0.5		1.3
Included in earnings		0.1	(0.1)	0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	1.5	(2.2)	3.0	(1.5)
Purchases		1.4	0.3	3.1
Sales	(1.4)	(2.5)	(2.9)	(3.9)
Ending Balance	60.8	60.8	60.8	60.8
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	55.8	58.4	55.6	56.7
Transfers into Level 3		0.5		1.3
Included in earnings		0.1	(0.1)	0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	1.5	(2.2)	3.0	(1.6)
Purchases		1.4	0.3	3.1
Sales	(1.4)	(2.5)	(2.9)	(3.9)
Ending Balance	55.9	55.7	55.9	55.7
Fixed Maturities [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	34.7	27.2	34.4	25.7
Included in earnings			0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.6	(1.2)	1.6	(0.8)
Purchases		1.4		3.1
Sales	(0.9)	(0.4)	(1.7)	(1.0)
Ending Balance	34.4	27.0	34.4	27.0
Fixed Maturities [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	3.8	9.5	3.7	9.6
Included in earnings		0.1	(0.2)	0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.7	(0.1)	0.7	(0.2)
Purchases			0.3
Sales	(0.1)	(0.1)	(0.1)	(0.1)
Ending Balance	4.4	9.4	4.4	9.4
Fixed Maturities [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	16.8	20.9	17.0	21.4
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.2	(0.9)	0.7	(0.6)
Sales	(0.3)	(2.0)	(1.0)	(2.8)
Ending Balance	16.7	18.0	16.7	18.0
Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.5	0.8	0.5
Transfers into Level 3		0.5		1.3
Sales	(0.1)		(0.1)
Ending Balance	0.4	1.3	0.4	1.3
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
Ending Balance	$ 4.9	$ 5.1	$ 4.9	$ 5.1