Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 33,900	$ 34,839
Provision (credit) for loan losses	4,465	3,753
Recoveries	1,341	1,017
Charge-offs	(2,882)	(4,621)
Transfer of off balance sheet exposure to other liabilities		(1,088)
Ending balance	36,824	33,900
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	24,940	24,063
Provision (credit) for loan losses	2,287	3,288
Recoveries	505	239
Charge-offs	(1,075)	(1,970)
Transfer of off balance sheet exposure to other liabilities		(680)
Ending balance	26,657	24,940
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	383	1,541
Provision (credit) for loan losses	128	(1,066)
Recoveries	18	39
Charge-offs	(2)	(100)
Transfer of off balance sheet exposure to other liabilities		(31)
Ending balance	527	383
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,440	7,343
Provision (credit) for loan losses	1,787	220
Recoveries	346	402
Charge-offs	(583)	(1,283)
Transfer of off balance sheet exposure to other liabilities		(242)
Ending balance	7,990	6,440
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	2,137	1,892
Provision (credit) for loan losses	263	1,311
Recoveries	472	337
Charge-offs	(1,222)	(1,268)
Transfer of off balance sheet exposure to other liabilities		(135)
Ending balance	$ 1,650	$ 2,137