Reconciliation for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 102.7	$ 102.5
Transfers into Level 3	9.9	4.4
Transfers out of Level 3	(2.4)	(7.7)
Included in earnings	0.4	(0.2)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	1.0	(0.7)
Purchases		8.2
Sales	(6.0)	(6.3)
Ending Balance	105.6	100.2
Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	74.7	75.5
Transfers into Level 3	9.9	4.3
Transfers out of Level 3	(1.5)	(7.7)
Included in earnings	0.4
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.5)	0.5
Purchases		8.2
Sales	(6.0)	(5.6)
Ending Balance	77.0	75.2
Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	19.4	13.6
Transfers into Level 3	9.7
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	0.1	0.6
Purchases		3.0
Sales	(0.4)	(0.2)
Ending Balance	28.8	17.0
Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	26.4	23.8
Transfers into Level 3	0.2	4.3
Included in earnings	0.4	0.1
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.1)	0.8
Sales	(4.1)	(0.3)
Ending Balance	22.8	28.7
Fixed Maturities | Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.7	5.2
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities		0.1
Sales	(0.1)	(4.6)
Ending Balance	0.6	0.7
Fixed Maturities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	26.7	23.7
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.5)	(1.0)
Purchases		5.2
Sales	(1.4)	(0.4)
Ending Balance	24.8	27.5
Fixed Maturities | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	1.5	9.2
Transfers out of Level 3	(1.5)	(7.7)
Included in earnings		(0.1)
Sales		(0.1)
Ending Balance		1.3
Equities and Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	28.0	27.0
Transfers into Level 3		0.1
Transfers out of Level 3	(0.9)
Included in earnings		(0.2)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	1.5	(1.2)
Sales		(0.7)
Ending Balance	$ 28.6	$ 25.0