Allowance for Loan Losses - Loans by Risk Rating (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Loans by risk rating
Financing Receivable, Net	$ 1,841,093	$ 1,731,536	$ 1,640,830
1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	1,395,294	1,276,651
4 (Monitor) [Member]
Loans by risk rating
Financing Receivable, Net	7,158	7,977
Impaired [Member]
Loans by risk rating
Financing Receivable, Net	4,201	2,219
Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	19,522	27,421	27,308
Construction and Land Development [Member] | 1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	12,453	20,281
Construction and Land Development [Member] | 4 (Monitor) [Member]
Loans by risk rating
Financing Receivable, Net	6,974	7,042
Construction and Land Development [Member] | Impaired [Member]
Loans by risk rating
Financing Receivable, Net	95	98
Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	549,290	452,235	378,154
Commercial and Industrial [Member] | 1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	548,891	451,774
Commercial and Industrial [Member] | 4 (Monitor) [Member]
Loans by risk rating
Financing Receivable, Net		18
Commercial and Industrial [Member] | Impaired [Member]
Loans by risk rating
Financing Receivable, Net	399	443
Municipal [Member]
Loans by risk rating
Financing Receivable, Net	145,063	85,685	87,016
Municipal [Member] | 1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	145,063	85,685
Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	692,778	721,506	$ 701,523
Commercial Real Estate [Member] | 1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	688,887	718,911
Commercial Real Estate [Member] | 4 (Monitor) [Member]
Loans by risk rating
Financing Receivable, Net	184	917
Commercial Real Estate [Member] | Impaired [Member]
Loans by risk rating
Financing Receivable, Net	$ 3,707	$ 1,678