Loans - Amounts of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage warehouse	$ 101,299	$ 94,508
Total loans	2,856,793	2,831,995
Allowance for Loan Losses	(16,474)	(16,394)	$ (15,054)	$ (14,837)
Loans, net	2,840,319	2,815,601
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	1,656,374	1,669,728
Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	618,131	606,760
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	480,989	460,999
Working Capital and Equipment [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	721,239	720,477
Real Estate Including Agriculture [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	865,279	880,861
Tax Exempt Loans Receivable [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	36,754	36,324
Other Commercial Loans [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	33,102	32,066
1-4 Family [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	610,763	599,217
Other Real Estate Loans [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	7,368	7,543
Auto [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	267,386	244,003
Recreation Consumer Loans Receivable [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	8,749	8,728
Real Estate Home Improvement Loans Receivable [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	36,073	37,052
Home Equity Loan [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	163,017	165,240
Unsecured Debt [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	3,257	3,479
Other Consumer Loans [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	$ 2,507	$ 2,497