Summary of Interest Recognized and Cash-Basis Interest Earned on Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Impaired [Line Items]
Average of individually impaired loans during period	$ 2,719	$ 2,690	$ 4,185
Interest income recognized during impairment	7	39	134
Cash-basis interest income recognized	$ 5	$ 39