Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 3,282,199	$ 3,317,050	$ 2,928,476
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	669,525	696,163	651,723
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	87,490	102,351	90,610
Real Estate Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,150,132	2,136,233	1,826,579
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	375,052	382,303	359,564
Pass [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	3,140,057	3,167,624	2,850,728
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	611,466	633,083	631,100
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	84,555	99,862	90,030
Pass [Member] | Real Estate Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,072,210	2,054,738	1,771,889
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	371,826	379,941	357,709
Special Mention [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	31,486	40,576	30,874
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	7,378	9,762	9,384
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	350	1,398	164
Special Mention [Member] | Real Estate Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	23,359	29,000	20,809
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	399	416	517
Substandard [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	110,656	108,813	46,775
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	50,681	53,318	11,239
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,585	1,091	416
Substandard [Member] | Real Estate Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	54,563	52,458	33,782
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 2,827	1,946	1,338
Doubtful [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan		37	99
Doubtful [Member] | Real Estate Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan		$ 37	$ 99