REGULATORY MATTERS - Regulatory Capital (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Regulatory matters
Capital conversion buffer (as a percent)	2.50%
Term of implementation of the capital conservation buffer	4 years
Common Equity Tier I to Risk‑Weighted Assets
Actual amount	$ 391,556	$ 361,322
Common Equity Tier One Capital Ratio	14.30%	14.20%
Minimum Capital Required for Capital Adequacy Purposes (amount)	$ 123,271	$ 114,628
Minimum Capital Required for Capital Adequacy Purposes (ratio)	4.50%	4.50%
Minimum Capital Required‑Basel III Fully Phased‑in (amount)	$ 191,755	$ 178,310
Minimum Capital Required‑Basel III Fully Phased‑in (ratio)	7.00%	7.00%
Tier I Capital to Risk‑Weighted Assets
Actual amount	$ 397,956	$ 367,722
Actual ratio	14.50%	14.40%
Minimum Capital Required for Capital Adequacy Purposes (amount)	$ 164,361	$ 152,837
Minimum Capital Required for Capital Adequacy Purposes (ratio)	6.00%	6.00%
Minimum Capital Required‑Basel III Fully Phased‑in (amount)	$ 232,845	$ 216,519
Minimum Capital Required‑Basel III Fully Phased‑in (ratio)	8.50%	8.50%
Total Capital to Risk‑Weighted Assets
Actual amount	$ 422,820	$ 392,878
Actual ratio	15.40%	15.40%
Minimum Capital Required for Capital Adequacy Purposes (amount)	$ 219,148	$ 203,782
Minimum Capital Required for Capital Adequacy Purposes (ratio)	8.00%	8.00%
Minimum Capital Required‑Basel III Fully Phased‑in (amount)	$ 287,632	$ 267,464
Minimum Capital Required‑Basel III Fully Phased‑in (ratio)	10.50%	10.50%
Leverage Ratio
Actual amount	$ 397,956	$ 367,722
Actual ratio	12.80%	12.30%
Minimum Capital Required for Capital Adequacy Purposes (amount)	$ 123,987	$ 119,769
Minimum Capital Required for Capital Adequacy Purposes (ratio)	4.00%	4.00%
Minimum Capital Required‑Basel III Fully Phased‑in (amount)	$ 123,987	$ 119,769
Minimum Capital Required‑Basel III Fully Phased‑in (ratio)	4.00%	4.00%
Bank Only
Common Equity Tier I to Risk‑Weighted Assets
Actual amount	$ 350,337	$ 322,414
Common Equity Tier One Capital Ratio	12.80%	12.70%
Minimum Capital Required for Capital Adequacy Purposes (amount)	$ 123,259	$ 114,252
Minimum Capital Required for Capital Adequacy Purposes (ratio)	4.50%	4.50%
Minimum Capital Required‑Basel III Fully Phased‑in (amount)	$ 182,549	$ 178,150
Minimum Capital Required‑Basel III Fully Phased‑in (ratio)	7.00%	7.00%
Required to be Considered Well Capitalized, amount	$ 178,040	$ 165,425
Required to be Considered Well Capitalized, ratio	6.50%	6.50%
Tier I Capital to Risk‑Weighted Assets
Actual amount	$ 350,337	$ 322,414
Actual ratio	12.80%	12.70%
Minimum Capital Required for Capital Adequacy Purposes (amount)	$ 164,345	$ 152,700
Minimum Capital Required for Capital Adequacy Purposes (ratio)	6.00%	6.00%
Minimum Capital Required‑Basel III Fully Phased‑in (amount)	$ 232,822	$ 216,325
Minimum Capital Required‑Basel III Fully Phased‑in (ratio)	8.50%	8.50%
Required to be Considered Well Capitalized, amount	$ 219,126	$ 203,600
Required to be Considered Well Capitalized, ratio	8.00%	8.00%
Total Capital to Risk‑Weighted Assets
Actual amount	$ 375,202	$ 347,569
Actual ratio	13.70%	13.70%
Minimum Capital Required for Capital Adequacy Purposes (amount)	$ 219,216	$ 203,600
Minimum Capital Required for Capital Adequacy Purposes (ratio)	8.00%	8.00%
Minimum Capital Required‑Basel III Fully Phased‑in (amount)	$ 287,604	$ 267,726
Minimum Capital Required‑Basel III Fully Phased‑in (ratio)	10.50%	10.50%
Required to be Considered Well Capitalized, amount	$ 273,908	$ 254,501
Required to be Considered Well Capitalized, ratio	10.00%	10.00%
Leverage Ratio
Actual amount	$ 350,336	$ 322,414
Actual ratio	11.30%	10.80%
Minimum Capital Required for Capital Adequacy Purposes (amount)	$ 123,987	$ 119,403
Minimum Capital Required for Capital Adequacy Purposes (ratio)	4.00%	4.00%
Minimum Capital Required‑Basel III Fully Phased‑in (amount)	$ 123,987	$ 119,403
Minimum Capital Required‑Basel III Fully Phased‑in (ratio)	4.00%	4.00%
Required to be Considered Well Capitalized, amount	$ 154,983	$ 149,253
Required to be Considered Well Capitalized, ratio	5.00%	5.00%
Minimum
Regulatory matters
Capital conversion buffer (as a percent)	0.625%