Loans - Amounts of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage warehouse	$ 133,428		$ 74,120
Total loans	3,664,668		3,013,332
Allowance for loan losses	(18,305)	$ (17,821)	(17,820)	$ (17,071)	$ (16,474)	$ (16,394)
Loans, net	3,646,363		2,995,512
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	2,062,623		1,721,590
Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	814,065		668,141
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	654,552		549,481
Working Capital and Equipment [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	917,533		804,083
Real Estate Including Agriculture [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	1,017,138		834,037
Tax Exempt Loans Receivable [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	61,015		48,975
Other Commercial Loans [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	66,937		34,495
1-4 Family [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	806,390		659,754
Other Real Estate Loans [Member] | Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real Estate, Total	7,675		8,387
Auto [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	343,876		327,413
Recreation Consumer Loans Receivable [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	16,257		13,975
Real Estate Home Improvement Loans Receivable [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	44,988		39,587
Home Equity Loan [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	239,358		163,209
Unsecured Debt [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	7,455		4,043
Other Consumer Loans [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial/Consumer, Total	$ 2,618		$ 1,254