Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Tri Countries Bank [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 525,384	$ 500,876
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	491,897	465,654
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	491,897	465,654
Tier 1 Capital (to Average Assets), Actual, Amount	$ 491,897	$ 465,654
Total Capital (to Risk Weighted Assets), Actual, Ratio	13.98%	14.71%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.09%	13.67%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.09%	13.66%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.73%	10.56%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 375,713	$ 340,529
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	300,570	274,725
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	244,214	221,344
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 229,243	$ 220,426
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	$ 347,535	$ 293,706
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	272,392	225,601
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	216,035	174,521
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 183,394	$ 176,341
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	9.25%	8.625%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.25%	6.625%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	5.75%	5.125%
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	$ 394,499	$ 357,556
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	319,356	289,450
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	262,999	238,370
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 183,394	$ 176,341
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%
Parent [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 528,805	$ 503,283
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	495,318	468,061
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	440,643	414,632
Tier 1 Capital (to Average Assets), Actual, Amount	$ 495,318	$ 468,061
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.07%	14.77%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.18%	13.74%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	11.72%	12.17%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.80%	10.62%
Parent [Member] | Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 347,694	$ 293,854
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	272,517	225,714
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	216,134	174,609
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 183,400	$ 176,346
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	9.25%	8.625%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.25%	6.625%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	5.75%	5.125%
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	$ 394,679	$ 357,735
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	319,502	289,595
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	263,120	238,490
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 183,400	$ 176,346
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%