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
	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments	$ 0.5	$ 1.9
Net realized investment gains (losses)	(0.4)	0.2
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments		(0.1)
Net realized investment gains (losses)	0.4	(0.1)
Total		(0.2)
Fair Value, Inputs, Level 3 | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments		(0.1)
Net realized investment gains (losses)	0.4	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)	0.4	0.1
Total		0.1
Fair Value, Inputs, Level 3 | Fixed Maturities | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments		(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]
Net realized investment gains (losses)		(0.2)
Total		$ (0.2)