Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 48,156	$ 45,779
Provision for loan losses	1,310	1,950
Recoveries	504	489
Charge-offs	(471)	(2,026)
Ending balance	49,499	46,192
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	10,865	11,707
Provision for loan losses	(1,627)	1,003
Recoveries	158	227
Charge-offs	(119)	(1,255)
Ending balance	9,277	11,682
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,305	1,101
Provision for loan losses	203	(153)
Recoveries	4	6
Charge-offs		(8)
Ending balance	1,512	946
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	29,896	26,864
Provision for loan losses	2,434	773
Recoveries	242	52
Charge-offs	(33)	(410)
Ending balance	32,539	27,279
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,090	6,107
Provision for loan losses	300	327
Recoveries	100	204
Charge-offs	(319)	(353)
Ending balance	$ 6,171	$ 6,285

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

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Amount of expense related to estimated loss from loan and lease transactions.

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