Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 3,472,227	$ 3,357,307	$ 3,337,793
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	674,947	674,410	663,581
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	83,005	84,021	84,716
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,297,556	2,189,844	2,191,260
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	416,719	409,032	398,236
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,350,933	3,229,505	3,211,193
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	632,693	629,756	614,900
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	79,227	81,620	82,400
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,224,970	2,111,947	2,118,807
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	414,043	406,182	395,086
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	35,237	24,787	25,488
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	7,997	5,769	6,108
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	841	715
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	26,231	18,091	19,064
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	168	212	316
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	86,057	103,015	101,110
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	34,257	38,885	42,573
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,937	1,686	2,316
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	46,355	59,806	53,389
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 2,508	$ 2,638	2,832
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan			2
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan			$ 2