Allowance for Loan Losses - Loans by Risk Rating (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Loans by risk rating
Financing Receivable, Net	$ 2,033,799	$ 1,923,933	$ 1,801,904
Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	14,159	14,928	31,178
Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	650,542	612,503	493,033
Municipal [Member]
Loans by risk rating
Financing Receivable, Net	127,259	135,418	134,386
Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	736,603	696,173	$ 728,658
1-3 (Pass) [Member] | Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	14,159	14,834
1-3 (Pass) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	650,177	612,114
1-3 (Pass) [Member] | Municipal [Member]
Loans by risk rating
Financing Receivable, Net	127,259	135,418
1-3 (Pass) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	704,531	661,271
4 (Monitor) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	29,469	31,753
Impaired [Member] | Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net		94
Impaired [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	365	389
Impaired [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	$ 2,603	$ 3,149