Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 4,232,048	$ 3,851,398
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 17,605	$ 20,431
Percent	0.42%	0.53%
Tax-exempt [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 33,575	$ 20,952
Percent	0.79%	0.54%
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 795,803	$ 745,106
Percent	18.80%	19.35%
Oil & Gas Production And Equipment [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 87,304	$ 104,940
Percent	2.06%	2.72%
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 150,620	$ 132,830
Percent	3.56%	3.45%
Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 403,664	$ 356,621
Percent	9.54%	9.26%
Farmland [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 184,707	$ 149,507
Percent	4.36%	3.88%
One to Four Family Residences [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 821,251	$ 766,362
Percent	19.41%	19.90%
Multifamily Residential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 65,477	$ 66,766
Percent	1.55%	1.73%
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 1,356,430	$ 1,191,477
Percent	32.05%	30.94%
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 283,636	$ 267,179
Percent	6.70%	6.94%
Other (Not Classified Above) [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 31,976	$ 29,227
Percent	0.76%	0.76%