Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Mar. 31, 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	$ 700,542	$ 682,419
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	665,688	647,262
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	610,317	591,933
Tier 1 Capital (to Average Assets), Actual, Amount	$ 665,688	$ 647,262
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.73%	14.40%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	14.00%	13.66%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	12.83%	12.49%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.84%	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	$ 449,380	$ 497,486
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	404,260	402,727
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	332,920	331,658
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 245,649	$ 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	$ 697,549	$ 680,624
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	662,695	645,467
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	662,695	645,467
Tier 1 Capital (to Average Assets), Actual, Amount	$ 662,695	$ 645,467
Total Capital (to Risk Weighted Assets), Actual, Ratio	14.67%	14.37%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.94%	13.63%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.94%	13.63%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.79%	10.65%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 475,424	$ 473,624
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	380,339	378,899
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	309,026	307,856
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	307,054	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	499,195	$ 497,305
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	404,111	402,581
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	332,797	331,537
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 245,643	$ 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%