Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Parent [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 503,283	$ 474,436
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	468,061	435,950
Tier 1 Capital (to Average Assets), Actual, Amount	$ 468,061	$ 435,950
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.65%	15.09%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.62%	13.86%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.62%	10.79%
Parent [Member] | Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 274,862	$ 251,555
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	206,147	188,666
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 176,346	$ 161,562
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	8.00%	8.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	6.00%	6.00%
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	$ 360,756	$ 330,165
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	292,041	267,277
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 176,346	$ 161,562
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%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio	4.00%	4.00%
Parent [Member] | Common Stock [Member]
Schedule of Capitalization [Line Items]
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	$ 414,632	$ 385,747
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	12.07%	12.27%
Parent [Member] | Common Stock [Member] | Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 154,610	$ 141,499
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	4.50%	4.50%
Parent [Member] | Common Stock [Member] | Basel III Fully Phased In [Member]
Schedule of Capitalization [Line Items]
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 240,504	$ 220,110
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.00%	7.00%
Tri Countries Bank [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 500,876	$ 473,327
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	465,654	434,841
Tier 1 Capital (to Average Assets), Actual, Amount	$ 465,654	$ 434,841
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.59%	15.06%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.56%	13.84%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.56%	10.76%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 343,407	$ 314,272
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	274,725	251,418
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 220,426	$ 202,002
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%
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	$ 274,725	$ 251,418
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	206,044	188,563
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 176,341	$ 161,601
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	8.00%	8.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	6.00%	6.00%
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	$ 360,577	$ 329,985
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	291,896	267,131
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 176,341	$ 161,601
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%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio	4.00%	4.00%
Tri Countries Bank [Member] | Common Stock [Member]
Schedule of Capitalization [Line Items]
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	$ 465,654	$ 434,841
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.56%	13.84%
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 223,214	$ 204,277
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tri Countries Bank [Member] | Common Stock [Member] | Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 154,533	$ 141,422
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	4.50%	4.50%
Tri Countries Bank [Member] | Common Stock [Member] | Basel III Fully Phased In [Member]
Schedule of Capitalization [Line Items]
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 240,385	$ 219,990
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.00%	7.00%