Summary of Gains and Losses Due to Changes in Fair Value in Net Income for Level Three Assets (Detail) (Fair Value, Inputs, Level 3, USD $) In Millions, unless otherwise specified	6 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Fixed Maturities	Jun. 30, 2012 Fixed Maturities	Jun. 30, 2012 Fixed Maturities Corporate	Jun. 30, 2012 Fixed Maturities Corporate	Jun. 30, 2012 Fixed Maturities Asset Backed Fixed Maturities	Jun. 30, 2012 Fixed Maturities Asset Backed Fixed Maturities	Jun. 30, 2012 Equity Securities	Jun. 30, 2011 Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other-than-temporary impairments	$ (0.1)	$ (0.5)		$ (0.1)				$ (0.1)		$ (0.5)
Net realized investment gains	0.2		0.3	0.4	0.2	0.3	0.1	0.1	(0.2)
Total	$ 0.1			$ 0.3		$ 0.3			$ (0.2)