Loans - Amounts of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage warehouse	$ 71,944	$ 74,120
Total loans	3,621,057	3,013,332
Allowance for loan losses	(17,821)	(17,820)	$ (16,474)	$ (16,394)
Loans, net	3,603,236	2,995,512
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	2,089,579	1,721,590
Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	819,824	668,141
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	639,710	549,481
Working Capital and Equipment [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	924,435	804,083
Real Estate Including Agriculture [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	1,076,077	834,037
Tax Exempt Loans Receivable [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	60,062	48,975
Other Commercial Loans [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	29,005	34,495
1-4 Family [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	808,401	659,754
Other Real Estate Loans [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	11,423	8,387
Auto [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	331,572	327,413
Recreation Consumer Loans Receivable [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	14,262	13,975
Real Estate Home Improvement Loans Receivable [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	40,998	39,587
Home Equity Loan [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	245,940	163,209
Unsecured Debt [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	4,124	4,043
Other Consumer Loans [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	$ 2,814	$ 1,254