Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 60.8	$ 67.3	$ 61.7	$ 104.2
Transfers into Level 3		2.2	1.3	4.4
Transfers out of Level 3	(4.6)		(4.6)	(40.0)
Included in earnings	0.1		0.2
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.2	(0.7)	(1.3)	0.8
Purchases	3.1		6.2	2.5
Sales	(0.9)	(1.1)	(4.8)	(4.2)
Ending Balance	58.7	67.7	58.7	67.7
Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	55.7	62.5	56.7	62.0
Transfers into Level 3		2.2	1.3	4.4
Transfers out of Level 3	(4.6)		(4.6)	(2.6)
Included in earnings	0.1		0.2
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.2	(0.7)	(1.4)	0.8
Purchases	3.1		6.2	2.5
Sales	(0.9)	(1.1)	(4.8)	(4.2)
Ending Balance	53.6	62.9	53.6	62.9
Fixed Maturities | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	27.0	26.8	25.7	25.6
Transfers into Level 3				2.2
Transfers out of Level 3				(2.6)
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.4	(0.2)	(0.4)	0.4
Purchases	3.1		6.2	2.5
Sales	(0.5)	(0.6)	(1.5)	(2.1)
Ending Balance	30.0	26.0	30.0	26.0
Fixed Maturities | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	9.4	13.2	9.6	13.0
Transfers into Level 3		2.2		2.2
Transfers out of Level 3	(4.6)		(4.6)
Included in earnings			0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities		(0.2)	(0.2)
Sales	(0.1)	(0.1)	(0.2)	(0.1)
Ending Balance	4.7	15.1	4.7	15.1
Fixed Maturities | Residential Mortgage Backed Securities, Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance				0.5
Sales				(0.5)
Fixed Maturities | Commercial Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	18.0	22.5	21.4	22.9
Included in earnings	0.1		0.1
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	(0.1)	(0.3)	(0.7)	0.4
Sales	(0.3)	(0.4)	(3.1)	(1.5)
Ending Balance	17.7	21.8	17.7	21.8
Fixed Maturities | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	1.3
Transfers into Level 3			1.3
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	(0.1)		(0.1)
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	5.1	4.8	5.0	42.2
Transfers out of Level 3				(37.4)
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities			0.1
Ending Balance	$ 5.1	$ 4.8	$ 5.1	$ 4.8