Loans - Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Ending allowance balance attributable to loans:
Individually evaluated for impairment	$ 168	$ 47
Collectively evaluated for impairment	14,982	13,957
Modified in a troubled debt restructuring	4,554	5,689
Total ending allowance balance	19,704	19,693	$ 16,330	$ 16,737
Loans:
Individually evaluated for impairment	3,292	1,238
Collectively evaluated for impairment	2,460,863	2,130,228
Total	2,479,348	2,149,848
Modified in a troubled debt restructuring	14,626	17,303
Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	567	1,079
Commercial, industrial, and agricultural
Ending allowance balance attributable to loans:
Individually evaluated for impairment	54	47
Collectively evaluated for impairment	7,183	5,868
Modified in a troubled debt restructuring	104	245
Total ending allowance balance	7,341	6,160	5,428	6,035
Loans:
Individually evaluated for impairment	1,334	1,187
Collectively evaluated for impairment	910,386	698,206
Total	916,297	704,606
Modified in a troubled debt restructuring	4,577	5,213
Commercial, industrial, and agricultural | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	0	0
Commercial mortgages
Ending allowance balance attributable to loans:
Individually evaluated for impairment	4	0
Collectively evaluated for impairment	3,036	3,563
Modified in a troubled debt restructuring	4,450	5,444
Total ending allowance balance	7,490	9,007	6,753	5,605
Loans:
Individually evaluated for impairment	1,446	51
Collectively evaluated for impairment	685,714	631,377
Total	697,776	644,597
Modified in a troubled debt restructuring	10,049	12,090
Commercial mortgages | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	567	1,079
Residential real estate
Ending allowance balance attributable to loans:
Individually evaluated for impairment	100	0
Collectively evaluated for impairment	2,056	2,033
Modified in a troubled debt restructuring	0	0
Total ending allowance balance	2,156	2,033	1,653	2,475
Loans:
Individually evaluated for impairment	502	0
Collectively evaluated for impairment	770,807	713,347
Total	771,309	713,347
Modified in a troubled debt restructuring	0	0
Residential real estate | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	0	0
Consumer
Ending allowance balance attributable to loans:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	2,377	2,179
Modified in a troubled debt restructuring	0	0
Total ending allowance balance	2,377	2,179	2,215	2,371
Loans:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	86,035	80,193
Total	86,035	80,193
Modified in a troubled debt restructuring	0	0
Consumer | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	0	0
Credit cards
Ending allowance balance attributable to loans:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	103	120
Modified in a troubled debt restructuring	0	0
Total ending allowance balance	103	120	93	90
Loans:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	7,623	6,753
Total	7,623	6,753
Modified in a troubled debt restructuring	0	0
Credit cards | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	0	0
Overdrafts
Ending allowance balance attributable to loans:
Individually evaluated for impairment	10	0
Collectively evaluated for impairment	227	194
Modified in a troubled debt restructuring	0	0
Total ending allowance balance	237	194	$ 188	$ 161
Loans:
Individually evaluated for impairment	10	0
Collectively evaluated for impairment	298	352
Total	308	352
Modified in a troubled debt restructuring	0	0
Overdrafts | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	$ 0	$ 0