Reconciliation of Activity for Assets Measured at Fair Value based on Significant Unobservable (Non-market) Information (Detail) (Bank-Issued Trust Preferred Securities, USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Bank-Issued Trust Preferred Securities
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance, January 1	$ 2,252	$ 2,619
Transfers into Level 3
Transfers out of Level 3
Other-than-temporary impairment loss included in earnings		(4)
Unrealized gain (loss) included in comprehensive income	679	(363)
Balance at September 30, 2012 and December 31, 2011	$ 2,931	$ 2,252