Capital Requirements and Restrictions on Retained Earnings - Summary of Actual and Required Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Actual Amount	$ 297,708	$ 261,530
Total Capital to Risk Weighted Assets, Actual Ratio	14.32%	14.05%
Total Capital to Risk Weighted Assets, For Capital Adequacy Purposes Amount	$ 192,341	$ 160,501
Total Capital to Risk Weighted Assets, For Capital Adequacy Purposes Ratio	9.25%	8.625%
Tier 1 (Core) Capital to Risk Weighted Assets, Actual Amount	$ 228,015	$ 195,200
Tier 1 (Core) Capital to Risk Weighted Assets, Actual Ratio	10.97%	10.49%
Tier 1 (Core) Capital to Risk Weighted Assets, For Capital Adequacy Purposes Amount	$ 150,754	$ 123,283
Tier 1 (Core) Capital to Risk Weighted Assets, For Capital Adequacy Purposes Ratio	7.25%	6.625%
Common equity Tier 1 to Risk Weighted Assets, Actual Amount	$ 208,015	$ 175,200
Common equity Tier 1 to Risk Weighted Assets, Actual Ratio	10.00%	9.41%
Common equity Tier 1 to Risk Weighted Assets, For Capital Adequacy Purposes Amount	$ 119,563	$ 95,370
Common equity Tier 1 to Risk Weighted Assets, For Capital Adequacy Purposes Ratio	5.75%	5.125%
Tier 1 (Core) Capital to Average Assets, Actual Amount	$ 228,015	$ 195,200
Tier 1 (Core) Capital to Average Assets, Actual Ratio	8.45%	7.85%
Tier 1 (Core) Capital to Average Assets, For Capital Adequacy Purposes, Amount	$ 107,969	$ 99,430
Tier 1 (Core) Capital to Average Assets, For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Actual Amount	$ 279,704	$ 242,592
Total Capital to Risk Weighted Assets, Actual Ratio	13.58%	13.19%
Total Capital to Risk Weighted Assets, For Capital Adequacy Purposes Amount	$ 190,513	$ 158,590
Total Capital to Risk Weighted Assets, For Capital Adequacy Purposes Ratio	9.25%	8.625%
Total Capital to Risk Weighted Assets, To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 205,960	$ 183,873
Total Capital to Risk Weighted Assets, To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	10.00%	10.00%
Tier 1 (Core) Capital to Risk Weighted Assets, Actual Amount	$ 261,643	$ 228,109
Tier 1 (Core) Capital to Risk Weighted Assets, Actual Ratio	12.70%	12.41%
Tier 1 (Core) Capital to Risk Weighted Assets, For Capital Adequacy Purposes Amount	$ 149,321	$ 121,816
Tier 1 (Core) Capital to Risk Weighted Assets, For Capital Adequacy Purposes Ratio	7.25%	6.625%
Tier 1 (Core) Capital to Risk Weighted Assets, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 164,768	$ 147,098
Tier 1 (Core) Capital to Risk Weighted Assets, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Common equity Tier 1 to Risk Weighted Assets, Actual Amount	$ 254,264	$ 220,730
Common equity Tier 1 to Risk Weighted Assets, Actual Ratio	12.35%	12.00%
Common equity Tier 1 to Risk Weighted Assets, For Capital Adequacy Purposes Amount	$ 118,427	$ 94,235
Common equity Tier 1 to Risk Weighted Assets, For Capital Adequacy Purposes Ratio	5.75%	5.125%
Common equity Tier 1 to Risk Weighted Assets, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 133,874	$ 119,517
Common equity Tier 1 to Risk Weighted Assets, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tier 1 (Core) Capital to Average Assets, Actual Amount	$ 261,643	$ 228,109
Tier 1 (Core) Capital to Average Assets, Actual Ratio	9.80%	9.36%
Tier 1 (Core) Capital to Average Assets, For Capital Adequacy Purposes, Amount	$ 106,798	$ 97,527
Tier 1 (Core) Capital to Average Assets, For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 (Core) Capital to Average Assets, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 133,498	$ 121,909
Tier 1 (Core) Capital to Average Assets, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%