Loans and Allowance for Loan Losses - Schedule of Loans Outstanding by Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 3,959,669	$ 3,851,398
Percent	100.00%	100.00%
Taxable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 17,674	$ 20,431
Percent	0.45%	0.53%
Tax-exempt [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 28,855	$ 20,952
Percent	0.73%	0.54%
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 759,935	$ 745,106
Percent	19.19%	19.35%
Oil & Gas Production And Equipment [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 68,943	$ 104,940
Percent	1.74%	2.72%
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 112,675	$ 132,830
Percent	2.84%	3.45%
Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 382,883	$ 356,621
Percent	9.67%	9.26%
Farmland [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 163,989	$ 149,507
Percent	4.14%	3.88%
One to Four Family Residences [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 797,974	$ 766,362
Percent	20.15%	19.90%
Multifamily Residential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 63,016	$ 66,766
Percent	1.59%	1.73%
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 1,250,673	$ 1,191,477
Percent	31.59%	30.94%
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 281,800	$ 267,179
Percent	7.12%	6.94%
Other (Not Classified Above) [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 31,252	$ 29,227
Percent	0.79%	0.76%