Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	1.25%	0.625%
Tri Countries Bank [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 522,718	$ 500,876
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	490,982	465,654
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	490,982	465,654
Tier 1 Capital (to Average Assets), Actual, Amount	$ 490,982	$ 465,654
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.35%	14.71%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.48%	13.67%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.48%	13.66%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.92%	10.56%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 364,278	$ 340,529
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	291,422	274,725
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	236,781	221,344
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 224,900	$ 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	$ 336,957	$ 293,706
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	264,102	225,601
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	209,460	174,521
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 179,920	$ 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	$ 382,492	$ 357,556
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	309,636	289,450
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	254,995	238,370
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 179,920	$ 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	$ 525,569	$ 503,283
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	493,833	468,061
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	439,498	414,632
Tier 1 Capital (to Average Assets), Actual, Amount	$ 493,833	$ 468,061
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.42%	14.77%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.55%	13.74%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	12.06%	12.17%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.98%	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	$ 337,117	$ 293,854
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	264,227	225,714
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	209,559	174,609
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 179,925	$ 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	$ 382,673	$ 357,735
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	309,783	289,595
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	255,116	238,490
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 179,925	$ 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%