Securities Sold Under Agreements to Repurchase - Securities Sold Under Agreements to Repurchase (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Debt Disclosure [Abstract]
Outstanding balance at end of period	$ 65,680,000	$ 64,765,000
Average interest rate at end of period	0.13%	0.13%
Average daily balance during the period	64,309,000	61,930,000
Average interest rate during the period	0.12%	0.25%
Maximum daily balance during the period	$ 76,979,000	$ 81,980,000