Regulatory Matters - Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Total Capital (to Risk-Weighted Assets):
Total Capital (to Risk-Weighted Assets) Actual, Amount	$ 563,797	$ 462,198
Total Capital (to Risk-Weighted Assets) Actual, Ratio	14.00%	12.44%
Total Capital (to Risk-Weighted Assets) For Capital Adequacy, Amount	$ 322,148	$ 297,284
Total Capital (to Risk-Weighted Assets) For Capital Adequacy, Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 402,685	$ 371,604
Total Capital (to Risk-Weighted Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets):
Tier 1 Capital (to Risk-Weighted Assets) Actual, Amount	$ 447,228	$ 349,942
Tier 1 Capital (to Risk-Weighted Assets) Actual, Ratio	11.11%	9.42%
Tier 1 Capital (to Risk - Weighed Assets) For Capital Adequacy, Amount	$ 241,611	$ 222,963
Tier 1 Capital (to Risk - Weighted Assets) For Capital Adequacy, Ratio	6.00%	6.00%
Tier 1 Capital (to Risk-Weighted Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 322,148	$ 297,284
Tier 1 Capital (to Risk-Weighted Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Tier 1 Common Capital (to Risk-Weighted Assets)
Tier One Common Risk Based Capital	$ 447,228	$ 349,942
Tier One Common Risk Based Capital To Risk Weighted Assets	11.11%	9.42%
Tier One Common Risk Based Capital Required For Capital Adequacy	$ 181,208	$ 167,222
Tier One Common Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets	4.50%	4.50%
Tier One Common Risk Based Capital Required To Be Well Capitalized	$ 261,745	$ 241,543
Tier One Common Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Tier 1 Capital (to Average Assets):
Tier 1 Capital (to Average Assets) Actual, Amount	$ 447,228	$ 349,942
Tier 1 Capital (to Average Assets) Actual, Ratio	10.48%	8.84%
Tier 1 Capital (to Average Assets) For Capital Adequacy, Amount	$ 170,753	$ 158,410
Tier 1 Capital (to Average Assets) For Capital Adequacy, Ratio	4.00%	4.00%
Tier 1 Capital (to Average Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 213,441	$ 198,013
Tier 1 Capital (to Average Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%
Bank [Member]
Total Capital (to Risk-Weighted Assets):
Total Capital (to Risk-Weighted Assets) Actual, Amount	$ 464,851	$ 436,435
Total Capital (to Risk-Weighted Assets) Actual, Ratio	11.62%	11.85%
Total Capital (to Risk-Weighted Assets) For Capital Adequacy, Amount	$ 320,003	$ 294,679
Total Capital (to Risk-Weighted Assets) For Capital Adequacy, Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 400,004	$ 368,349
Total Capital (to Risk-Weighted Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets):
Tier 1 Capital (to Risk-Weighted Assets) Actual, Amount	$ 442,613	$ 418,266
Tier 1 Capital (to Risk-Weighted Assets) Actual, Ratio	11.07%	11.36%
Tier 1 Capital (to Risk - Weighed Assets) For Capital Adequacy, Amount	$ 240,002	$ 221,010
Tier 1 Capital (to Risk - Weighted Assets) For Capital Adequacy, Ratio	6.00%	6.00%
Tier 1 Capital (to Risk-Weighted Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 320,003	$ 294,679
Tier 1 Capital (to Risk-Weighted Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Tier 1 Common Capital (to Risk-Weighted Assets)
Tier One Common Risk Based Capital	$ 442,613	$ 418,266
Tier One Common Risk Based Capital To Risk Weighted Assets	11.07%	11.36%
Tier One Common Risk Based Capital Required For Capital Adequacy	$ 180,002	$ 165,757
Tier One Common Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets	4.50%	4.50%
Tier One Common Risk Based Capital Required To Be Well Capitalized	$ 260,002	$ 239,427
Tier One Common Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Tier 1 Capital (to Average Assets):
Tier 1 Capital (to Average Assets) Actual, Amount	$ 442,613	$ 418,266
Tier 1 Capital (to Average Assets) Actual, Ratio	10.45%	10.64%
Tier 1 Capital (to Average Assets) For Capital Adequacy, Amount	$ 169,453	$ 157,254
Tier 1 Capital (to Average Assets) For Capital Adequacy, Ratio	4.00%	4.00%
Tier 1 Capital (to Average Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 211,816	$ 196,567
Tier 1 Capital (to Average Assets) To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%