Loans and Allowance for Loan Losses - Schedule of Aging of Recorded Investment in Past Due Loans by Segment and Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable Recorded Investment Past Due [Line Items]
Greater Than 90 Days Past Due Still On Accrual		$ 18
Nonaccrual	$ 71,047	33,203
Loans Not Past Due	2,540,659	2,533,803
Total Loans	2,618,986	2,575,408
30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	5,291	3,280
60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	1,989	5,104
Commercial Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	13,275	12,768
Loans Not Past Due	1,218,518	1,216,888
Total Loans	1,234,186	1,231,217
Commercial Real Estate [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	2,168	1,302
Commercial Real Estate [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	225	259
Commercial and Industrial [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	34,126	6,954
Loans Not Past Due	552,750	591,852
Total Loans	587,884	601,782
Commercial and Industrial [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	272	509
Commercial and Industrial [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	736	2,467
Residential Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Greater Than 90 Days Past Due Still On Accrual		18
Nonaccrual	16,013	5,257
Loans Not Past Due	491,660	437,815
Total Loans	510,092	446,060
Residential Real Estate [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	1,791	782
Residential Real Estate [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	628	2,188
Agricultural Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	5,202	4,857
Loans Not Past Due	129,855	134,445
Total Loans	135,489	139,332
Agricultural Real Estate [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	432
Agricultural Real Estate [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due		30
Consumer [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	812	914
Loans Not Past Due	70,262	61,322
Total Loans	71,753	62,894
Consumer [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	611	501
Consumer [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	68	157
Agricultural [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	1,619	2,453
Loans Not Past Due	77,614	91,481
Total Loans	79,582	94,123
Agricultural [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	17	186
Agricultural [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	$ 332	$ 3