Debt Securities for which Credit Loss was Recognized in Income and Other Losses Recorded in Other Comprehensive Income (Detail) (USD $) In Thousands, 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]
Credit losses on debt securities held, beginning of year	$ 3,567	$ 3,374	$ 2,957
Reductions related to actual losses incurred	(2,362)
Additions related to increases in previously recognized other-than-temporary losses		193	417
Credit losses on debt securities held, end of period	$ 1,205	$ 3,567	$ 3,374