Regulatory Matters - Actual and Required Capital Ratios of Bank (Detail) - USD ($) $ in Thousands	Mar. 31, 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%
Basel III Phase-in Schedule [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Minimum Capital Requirement, Ratio	1.875%	1.25%
Parent [Member]
Schedule of Capitalization [Line Items]
Total Capital (to Risk Weighted Assets), Actual, Amount	$ 539,966	$ 528,805
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	506,129	495,318
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	450,933	440,643
Tier 1 Capital (to Average Assets), Actual, Amount	$ 506,129	$ 495,318
Total Capital (to Risk Weighted Assets), Actual, Ratio	13.91%	14.07%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	13.03%	13.18%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	11.61%	11.72%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.84%	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	$ 383,434	$ 347,694
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	305,777	272,517
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	247,534	216,134
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 186,720	$ 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	$ 407,702	$ 394,679
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	330,045	319,502
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	271,802	263,120
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 186,720	$ 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	$ 536,894	$ 525,384
Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	503,057	491,897
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Amount	503,057	491,897
Tier 1 Capital (to Average Assets), Actual, Amount	$ 503,057	$ 491,897
Total Capital (to Risk Weighted Assets), Actual, Ratio	13.83%	13.98%
Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	12.96%	13.09%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Actual, Ratio	12.96%	13.09%
Tier 1 Capital (to Average Assets), Actual, Ratio	10.78%	10.73%
Total Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 388,112	$ 375,713
Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	310,490	300,570
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	252,273	244,214
Tier 1 Capital (to Average Assets), Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	233,395	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	383,261	347,535
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	305,638	272,392
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	247,421	216,035
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 186,716	$ 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	$ 407,518	$ 394,499
Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	329,895	319,356
Common Equity Tier 1 Capital (to Risk Weighted Assets), Minimum Capital Requirement, Amount	271,678	262,999
Tier 1 Capital (to Average Assets), Minimum Capital Requirement, Amount	$ 186,716	$ 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%