Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount		$ 279,311
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount		167,587
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount		$ 185,918
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio		10.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio		6.00%
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio		5.00%
Parent [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 465,201	$ 436,955
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	426,816	401,971
Tier 1 Capital (to Average Assets), Actual, Amount	$ 426,816	$ 401,971
Total Capital (to Risk Weighted Assets), Actual, Ratio	15.17%	15.63%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.92%	14.38%
Tier 1 Capital (to Average Assets), Actual, Ratio	11.00%	10.80%
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount		$ 223,603
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount		111,801
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount		$ 148,819
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		8.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		4.00%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio		4.00%
Parent [Member] | Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	$ 245,283
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	183,962
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 155,201
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	8.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	6.00%
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	$ 321,934
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	260,613
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 155,201
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	10.50%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	8.50%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio	4.00%
Parent [Member] | Common Stock [Member]
Schedule of Capitalization [Line Items]
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	$ 376,661
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	12.28%
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	$ 137,972
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	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	$ 214,623
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.00%
Tri Countries Bank [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 463,445	$ 433,286
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	425,081	398,325
Tier 1 Capital (to Average Assets), Actual, Amount	$ 425,081	$ 398,325
Total Capital (to Risk Weighted Assets), Actual, Ratio	15.12%	15.51%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.87%	14.26%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.96%	10.71%
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount		$ 223,449
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount		111,724
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount		$ 148,734
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		8.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio		4.00%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio		4.00%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 306,427
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	245,142
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 193,909
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	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	$ 245,142
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	183,856
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 155,127
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	8.00%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	6.00%
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	$ 321,749
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	260,465
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 155,127
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	10.50%
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	8.50%
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Ratio	4.00%
Tri Countries Bank [Member] | Common Stock [Member]
Schedule of Capitalization [Line Items]
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	$ 425,081
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.87%
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 199,178
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	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	$ 137,892
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	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	$ 214,499
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	7.00%