Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 3,266,817	$ 2,929,188	$ 2,828,430
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	698,406	639,954	612,092
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	99,232	105,694	86,718
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,088,002	1,822,854	1,774,639
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	381,177	360,686	354,981
Pass [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	3,154,699	2,856,206	2,747,040
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	656,566	626,266	597,717
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	98,180	105,101	86,272
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,020,556	1,765,886	1,710,201
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	379,397	358,953	352,850
Special Mention [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	43,284	19,453	23,084
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	19,242	3,853	3,649
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	148	91	93
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	23,542	15,106	18,741
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	352	403	601
Substandard [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	68,772	53,388	58,219
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	22,598	9,835	10,726
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	904	502	353
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	43,846	41,722	45,613
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	1,424	1,329	1,527
Doubtful [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	62	141	87
Doubtful [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	58	140	84
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 4	$ 1	$ 3