Before-Tax Credit Related Portion of Other-Than-Temporary Impairments on Fixed Maturity Securities Still Held and Recognized in Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Balance at Beginning of Year	$ 0	$ 8.5	$ 18.3
Sales or Maturities of Securities in the Period	0	8.5	9.8
Balance at End of Year	$ 0	$ 0	$ 8.5