Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Capitalization [Line Items]
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%
Parent [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 550,769	$ 528,805
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	517,518	495,318
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	462,278	440,643
Tier 1 Capital (to Average Assets), Actual, Amount	$ 517,518	$ 495,318
Total Capital (to Risk Weighted Assets), Actual, Ratio	13.91%	14.07%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.07%	13.18%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	11.68%	11.72%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.92%	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	$ 390,991	$ 347,694
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	311,803	272,517
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	252,412	216,134
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 189,647	$ 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	$ 415,737	$ 394,679
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	336,549	319,502
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	277,158	263,120
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 189,647	$ 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	$ 548,232	$ 525,384
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	514,981	491,897
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	514,981	491,897
Tier 1 Capital (to Average Assets), Actual, Amount	$ 514,981	$ 491,897
Total Capital (to Risk Weighted Assets), Actual, Ratio	13.85%	13.98%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.01%	13.09%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.01%	13.09%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.86%	10.73%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 395,766	$ 375,713
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	316,613	300,570
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	257,248	244,214
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	237,054	229,243
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	390,819	347,535
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	311,666	272,392
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	252,301	216,035
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 189,643	$ 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	$ 415,554	$ 394,499
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	336,401	319,356
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	277,036	262,999
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 189,643	$ 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%