Summary of Gains and Losses Due to Changes in Fair Value in Net Income for Level Three Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments	$ 2.2	$ 1.5	$ 5.7	$ 3.7
Net realized investment gains (losses)	(0.1)	(0.1)	(0.2)	0.4
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments				(0.5)
Net realized investment gains (losses)			0.2	(0.1)
Total				(0.4)
Fair Value, Inputs, Level 3 | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net realized investment gains (losses)	0.1	0.1
Fair Value, Inputs, Level 3 | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net realized investment gains (losses)	0.1	0.1	0.4	(0.1)
Total				0.1
Fair Value, Inputs, Level 3 | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments				(0.5)
Net realized investment gains (losses)			(0.2)
Total				$ (0.5)