Activity Related to Credit Component Recognized in Earnings on Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Balance, beginning of year	$ 578	$ 330	$ 176
Additions to the credit component on debt securities in which other-than-temporary impairment was not previously recognized		248	154
Reductions due to sales	(578)
Cumulative pre-tax credit losses, end of year		$ 578	$ 330