Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Parent [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 682,419	$ 528,805
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	647,262	495,318
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	591,933	440,643
Tier 1 Capital (to Average Assets), Actual, Amount	$ 647,262	$ 495,318
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.40%	14.07%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.66%	13.18%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	12.49%	11.72%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.68%	10.80%
Parent [Member] | Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 467,874	$ 347,694
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	373,115	272,517
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	302,045	216,134
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 242,452	$ 183,400
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	9.875%	9.25%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.875%	7.25%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	6.375%	5.75%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio	4.00%	4.00%
Parent [Member] | Basel III Fully Phased In [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 497,486	$ 394,679
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	402,727	319,502
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	331,658	263,120
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 242,452	$ 183,400
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	10.50%	10.50%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	8.50%	8.50%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.00%	7.00%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio	4.00%	4.00%
Tri Countries Bank [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 680,624	$ 525,384
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	645,467	491,897
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	645,467	491,897
Tier 1 Capital (to Average Assets), Actual, Amount	$ 645,467	$ 491,897
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.37%	13.98%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.63%	13.09%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.63%	13.09%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.65%	10.73%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 473,624	$ 375,713
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	378,899	300,570
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	307,856	244,214
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 303,059	$ 229,243
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%
Tri Countries Bank [Member] | Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 467,704	$ 347,535
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	372,979	272,392
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	301,935	216,035
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 242,447	$ 183,394
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	9.875%	9.25%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.875%	7.25%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	6.375%	5.75%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio	4.00%	4.00%
Tri Countries Bank [Member] | Basel III Fully Phased In [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 497,305	$ 394,499
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	402,581	319,356
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	331,537	262,999
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 242,447	$ 183,394
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	10.50%	10.50%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	8.50%	8.50%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.00%	7.00%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio	4.00%	4.00%