Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 3,357,307	$ 3,317,050
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	674,410	696,163
Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	84,021	102,351
Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,189,844	2,136,233
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	409,032	382,303
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	3,229,505	3,167,624
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	629,756	633,083
Pass [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	81,620	99,862
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	2,111,947	2,054,738
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	406,182	379,941
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	24,787	40,576
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	5,769	9,762
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	715	1,398
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	18,091	29,000
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	212	416
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	103,015	108,813
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	38,885	53,318
Substandard [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	1,686	1,091
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	59,806	52,458
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan	$ 2,638	1,946
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan		37
Doubtful [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Internal ratings of loan		$ 37