Loans and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Receivables [Abstract]
Commercial	$ 651,723	$ 639,954	$ 607,281
Agricultural	90,610	105,694	65,121
Real estate	1,826,579	1,822,854	1,680,807
Consumer	359,564	360,686	335,079
Total	$ 2,928,476	$ 2,929,188	$ 2,688,288