Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
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	$ 718,901	$ 682,419
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	683,043	647,262
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	627,627	591,933
Tier 1 Capital (to Average Assets), Actual, Amount	$ 683,043	$ 647,262
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.93%	14.40%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	14.19%	13.66%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.03%	12.49%
Tier 1 Capital (to Average Assets), Actual, Ratio	11.08%	10.68%
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
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount		373,115
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount		302,045
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount		$ 242,452
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		9.875%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		7.875%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		6.375%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio		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	$ 505,569	$ 497,486
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	409,270	402,727
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	337,046	331,658
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 246,599	$ 242,452
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	$ 714,018	$ 680,624
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	678,160	645,467
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	678,160	645,467
Tier 1 Capital (to Average Assets), Actual, Amount	$ 678,160	$ 645,467
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.83%	14.37%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	14.09%	13.63%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	14.09%	13.63%
Tier 1 Capital (to Average Assets), Actual, Ratio	11.00%	10.65%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 481,320	$ 473,624
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	385,056	378,899
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	312,858	307,856
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	308,242	303,059
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
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount		372,979
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount		301,935
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount		$ 242,447
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		9.875%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		7.875%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		6.375%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio		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	505,385	$ 497,305
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	409,122	402,581
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	336,924	331,537
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 246,594	$ 242,447
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%