Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 41,877	$ 36,824
Provision for loan losses	10,212	9,685
Recoveries	3,506	1,407
Charge-offs	(9,816)	(6,039)
Ending balance	45,779	41,877
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	12,644	7,990
Provision for loan losses	5,101	8,044
Recoveries	952	344
Charge-offs	(6,990)	(3,734)
Ending balance	11,707	12,644
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,191	527
Provision for loan losses	104	773
Recoveries	25	55
Charge-offs	(219)	(164)
Ending balance	1,101	1,191
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	24,375	26,657
Provision for loan losses	1,150	(2,399)
Recoveries	2,021	558
Charge-offs	(682)	(441)
Ending balance	26,864	24,375
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	3,667	1,650
Provision for loan losses	3,857	3,267
Recoveries	508	450
Charge-offs	(1,925)	(1,700)
Ending balance	$ 6,107	$ 3,667