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	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Credit losses on debt securities held, beginning of year	$ (1,205)	$ (1,205)	$ (1,205)	$ (1,179)
Additions related to increases in previously recognized other-than-temporary losses				(26)
Credit losses on debt securities held, end of period	$ (1,205)	$ (1,205)	$ (1,205)	$ (1,205)