Loans - Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Ending allowance balance attributable to loans:
Individually evaluated for impairment	$ 1,943	$ 168
Collectively evaluated for impairment	17,172	14,982
Modified in a troubled debt restructuring	358	4,554
Total ending allowance balance	19,473	19,704	$ 19,693	$ 16,330
Loans:
Individually evaluated for impairment	10,953	3,292
Collectively evaluated for impairment	2,787,919	2,460,863
Total	2,809,197	2,479,348
Modified in a troubled debt restructuring	9,802	14,626
Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	523	567
Commercial, industrial, and agricultural
Ending allowance balance attributable to loans:
Individually evaluated for impairment	645	54
Collectively evaluated for impairment	7,614	7,183
Modified in a troubled debt restructuring	28	104
Total ending allowance balance	8,287	7,341	6,160	5,428
Loans:
Individually evaluated for impairment	8,078	1,334
Collectively evaluated for impairment	1,035,494	910,386
Total	1,046,665	916,297
Modified in a troubled debt restructuring	3,093	4,577
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	1,264	4
Collectively evaluated for impairment	5,358	3,036
Modified in a troubled debt restructuring	330	4,450
Total ending allowance balance	6,952	7,490	9,007	6,753
Loans:
Individually evaluated for impairment	2,410	1,446
Collectively evaluated for impairment	804,360	685,714
Total	814,002	697,776
Modified in a troubled debt restructuring	6,709	10,049
Commercial mortgages | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Total ending allowance balance	0	0
Loans:
Total	523	567
Residential real estate
Ending allowance balance attributable to loans:
Individually evaluated for impairment	34	100
Collectively evaluated for impairment	1,465	2,056
Modified in a troubled debt restructuring	0	0
Total ending allowance balance	1,499	2,156	2,033	1,653
Loans:
Individually evaluated for impairment	465	502
Collectively evaluated for impairment	813,565	770,807
Total	814,030	771,309
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	2,411	2,377
Modified in a troubled debt restructuring	0	0
Total ending allowance balance	2,411	2,377	2,179	2,215
Loans:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	124,785	86,035
Total	124,785	86,035
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	84	103
Modified in a troubled debt restructuring	0	0
Total ending allowance balance	84	103	120	93
Loans:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	7,569	7,623
Total	7,569	7,623
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	0	10
Collectively evaluated for impairment	240	227
Modified in a troubled debt restructuring	0	0
Total ending allowance balance	240	237	$ 194	$ 188
Loans:
Individually evaluated for impairment	0	10
Collectively evaluated for impairment	2,146	298
Total	2,146	308
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