Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 3,337,793	$ 3,317,050	$ 3,266,817
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	663,581	696,163	698,406
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	84,716	102,351	99,232
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,191,260	2,136,233	2,088,002
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	398,236	382,303	381,177
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,211,193	3,167,624	3,154,699
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	614,900	633,083	656,566
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	82,400	99,862	98,180
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,118,807	2,054,738	2,020,556
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	395,086	379,941	379,397
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	25,488	40,576	43,284
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	6,108	9,762	19,242
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan		1,398	148
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	19,064	29,000	23,542
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	316	416	352
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	101,110	108,813	68,772
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	42,573	53,318	22,598
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,316	1,091	904
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	53,389	52,458	43,846
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,832	1,946	1,424
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2	37	62
Doubtful [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan		$ 37	58
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 2		$ 4