Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 3,372,512	$ 3,357,307	$ 3,282,199
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	666,687	674,410	669,525
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	79,237	84,021	87,490
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,209,021	2,189,844	2,150,132
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	417,567	409,032	375,052
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,249,891	3,229,505	3,140,057
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	623,429	629,756	611,466
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	77,050	81,620	84,555
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,134,703	2,111,947	2,072,210
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	414,709	406,182	371,826
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	23,341	24,787	31,486
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	5,135	5,769	7,378
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	730	715	350
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	17,310	18,091	23,359
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	166	212	399
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	99,280	103,015	110,656
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	38,123	38,885	50,681
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	1,457	1,686	2,585
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	57,008	59,806	54,563
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 2,692	$ 2,638	$ 2,827