Loans - Aging of Recorded Investment in Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 17,153	$ 19,129
Loans Not Past Due	2,792,044	2,460,219
Total	2,809,197	2,479,348
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	5,278	7,608
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,366	4,618
Greater Than 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	9,509	6,903
Commercial, industrial, and agricultural
Financing Receivable, Past Due [Line Items]
Total Past Due	5,605	4,612
Loans Not Past Due	1,041,060	911,685
Total	1,046,665	916,297
Commercial, industrial, and agricultural | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,273	2,339
Commercial, industrial, and agricultural | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	548	9
Commercial, industrial, and agricultural | Greater Than 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,784	2,264
Commercial mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	2,939	4,096
Loans Not Past Due	811,063	693,680
Total	814,002	697,776
Commercial mortgages | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	162	758
Commercial mortgages | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	183	3,055
Commercial mortgages | Greater Than 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,594	283
Residential real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	7,367	9,227
Loans Not Past Due	806,663	762,082
Total	814,030	771,309
Residential real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,383	3,982
Residential real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,270	1,257
Residential real estate | Greater Than 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,714	3,988
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	1,138	1,115
Loans Not Past Due	123,647	84,920
Total	124,785	86,035
Consumer | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	412	470
Consumer | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	311	282
Consumer | Greater Than 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	415	363
Credit cards
Financing Receivable, Past Due [Line Items]
Total Past Due	104	79
Loans Not Past Due	7,465	7,544
Total	7,569	7,623
Credit cards | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	48	59
Credit cards | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	54	15
Credit cards | Greater Than 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2	5
Overdrafts
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Loans Not Past Due	2,146	308
Total	2,146	308
Overdrafts | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Overdrafts | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Overdrafts | Greater Than 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 0	$ 0