Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans held for sale (included above)	$ 5,231	$ 6,469	$ 7,211
Total loans	3,542,270	3,387,146	3,219,967
Percent	100.00%	100.00%	100.00%
Taxable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	9,824	10,217	9,272
Percent	0.28%	0.30%	0.29%
Tax-exempt [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	11,219	11,073	13,034
Percent	0.32%	0.33%	0.41%
Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	676,084	605,672	518,438
Percent	19.09%	17.88%	16.10%
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	109,570	111,323	96,094
Percent	3.09%	3.29%	2.98%
Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	299,238	284,808	231,770
Percent	8.45%	8.41%	7.20%
Farmland [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	141,059	132,512	124,347
Percent	3.98%	3.91%	3.86%
One to four family residences [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	723,358	703,903	680,129
Percent	20.42%	20.78%	21.12%
Multifamily residential properties [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	60,785	60,080	47,506
Percent	1.72%	1.77%	1.48%
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,134,384	1,097,484	1,084,864
Percent	32.02%	32.40%	33.69%
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	251,651	250,588	240,600
Percent	7.10%	7.40%	7.47%
Other (not classified above) [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	25,716	22,579	19,521
Percent	0.73%	0.67%	0.61%
Oil & gas production & equipment [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 99,382	$ 96,907	$ 154,392
Percent	2.80%	2.86%	4.79%

Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) (USD $)
In Thousands, unless otherwise specified

Mar. 31, 2014

Dec. 31, 2013

Mar. 31, 2013

Financing Receivable, Allowance for Credit Losses [Line Items]

Loans held for sale (included above)

$ 5,231

$ 6,469

$ 7,211

Total loans

3,542,270

3,387,146