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	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments	$ 7.8	$ 6.9	$ 13.9
Net realized investment gains (losses)	(0.2)	2.1
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments		(2.3)
Net realized investment gains (losses)	0.2	0.2
Total		(2.1)
Fair Value, Inputs, Level 3 | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments		(0.9)
Net realized investment gains (losses)	0.4	0.2
Total		(0.7)
Fair Value, Inputs, Level 3 | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net realized investment gains (losses)		(0.1)
Total		(0.1)
Fair Value, Inputs, Level 3 | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments		(0.9)
Net realized investment gains (losses)	0.4	0.2
Total		(0.7)
Fair Value, Inputs, Level 3 | Fixed Maturities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net realized investment gains (losses)		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		(1.4)
Net realized investment gains (losses)	(0.2)
Total		$ (1.4)