LOAN PERFORMANCE - Aging analysis of loan past due (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Aging analysis
Total past due	$ 10,056		$ 10,056		$ 6,857
Total current loans	2,194,467		2,194,467		2,153,189
Total loans	2,204,523		2,204,523		2,160,046
Interest income that would have been earned under the original terms of the nonaccrual loans	204	$ 326	314	$ 537
30 to 59 days past due
Aging analysis
Total past due	4,225		4,225		1,129
60 to 89 days past due
Aging analysis
Total past due	999		999		833
90 days or greater past due
Aging analysis
Total past due	4,832		4,832		4,895
Commercial and industrial
Aging analysis
Total past due	2,988		2,988		2,719
Total current loans	545,882		545,882		508,835
Total loans	548,870		548,870		511,554
Commercial and industrial | 30 to 59 days past due
Aging analysis
Total past due	1,153		1,153		378
Commercial and industrial | 60 to 89 days past due
Aging analysis
Total past due	531		531		68
Commercial and industrial | 90 days or greater past due
Aging analysis
Total past due	1,304		1,304		2,273
Commercial real estate
Aging analysis
Total past due	4,259		4,259		2,292
Total current loans	685,242		685,242		695,502
Total loans	689,501		689,501		697,794
Commercial real estate | 30 to 59 days past due
Aging analysis
Total past due	667		667		352
Commercial real estate | 60 to 89 days past due
Aging analysis
Total past due	64		64		75
Commercial real estate | 90 days or greater past due
Aging analysis
Total past due	3,528		3,528		1,865
Construction and development
Aging analysis
Total past due	102		102		19
Total current loans	424,387		424,387		491,607
Total loans	424,489		424,489		491,626
Construction and development | 30 to 59 days past due
Aging analysis
Total past due	102		102		19
One to four family residential
Aging analysis
Total past due	2,417		2,417		1,822
Total current loans	244,147		244,147		235,060
Total loans	246,564		246,564		236,882
One to four family residential | 30 to 59 days past due
Aging analysis
Total past due	2,013		2,013		377
One to four family residential | 60 to 89 days past due
Aging analysis
Total past due	404		404		688
One to four family residential | 90 days or greater past due
Aging analysis
Total past due					757
Multi‑family residential
Aging analysis
Total past due	41		41
Total current loans	211,178		211,178		133,210
Total loans	211,219		211,219		133,210
Multi‑family residential | 30 to 59 days past due
Aging analysis
Total past due	41		41
Consumer
Aging analysis
Total past due	47		47
Total current loans	42,725		42,725		39,694
Total loans	42,772		42,772		39,694
Consumer | 30 to 59 days past due
Aging analysis
Total past due	47		47
Agriculture
Aging analysis
Total current loans	11,424		11,424		11,106
Total loans	11,424		11,424		11,106
Other
Aging analysis
Total past due	202		202		5
Total current loans	29,482		29,482		38,175
Total loans	29,684		29,684		38,180
Other | 30 to 59 days past due
Aging analysis
Total past due	$ 202		$ 202		3
Other | 60 to 89 days past due
Aging analysis
Total past due					$ 2