Acquisition - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended				9 Months Ended	12 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Jul. 17, 2012	Sep. 30, 2013 Heartland [Member]	Dec. 31, 2012 Heartland [Member]	Jul. 17, 2012 Heartland [Member]	Sep. 30, 2013 Heartland Bancshares Inc [Member]
Business Acquisition [Line Items]
Date of acquisition agreement			Jul. 17, 2012
Exchange ratio per share to Heartland shareholder									$ 0.81
Common stock outstanding	8,619,716		8,619,716		8,617,466				1,442,449
Common stock issued							1,168,383
Market closing price per share									$ 16.83
Transaction value is estimated									$ 26,900,000
Costs related to the acquisition								1,500,000
Net tangible assets acquired									26,900,000
Net intangible assets acquired						2,332,000			2,300,000
Acquisition of goodwill						13,838,000			13,800,000
Purchase price of the business assets						2,647,000			10,800,000
Core deposit intangible amortization period							7 years
Loan portfolio acquired									131,100,000
Discount on loan portfolio acquired									16,500,000
Performing portion of the loan portfolio acquired									95,400,000
Estimated fair value of performing portion of the loan portfolio									91,600,000
Total provision charged to operating expense	104,000	1,041,000	2,917,000	1,809,000						1,600,000
Allowances for loan losses										$ 0