Changes in Amount of Credit Losses on Investment Securities Recognized In Earnings (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]
Beginning balance of credit losses previously recognized in earnings	$ 600	$ 400	$ 600	$ 400
Amount related to credit loss for securities as to which an other-than-temporary impairment was not previously recognized in earnings	0	0	0	0
Amount related to credit loss for securities as to which an other-than-temporary impairment was recognized in earnings	0	50	0	50
Ending balance of cumulative credit losses recognized in earnings	$ 600	$ 450	$ 600	$ 450