LOAN PERFORMANCE - Aging analysis of loan past due (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Aging analysis
Total past due	$ 3,399,000		$ 8,108,000
Total current loans	2,646,479,000		2,445,021,000
Recorded Investment with Allowance	2,649,878,000		2,453,129,000
90 days past due and still accruing	9,000
Interest income that would have been earned under the original terms of the nonaccrual loans	87,000	$ 139,000
30 to 59 days past due
Aging analysis
Total past due	1,166,000		1,028,000
60 to 89 days past due
Aging analysis
Total past due	407,000		6,285,000
90 days or greater past due
Aging analysis
Total past due	1,826,000		795,000
Commercial and industrial
Aging analysis
Total past due	1,771,000		1,213,000
Total current loans	538,313,000		518,566,000
Recorded Investment with Allowance	540,084,000		519,779,000
Commercial and industrial | 30 to 59 days past due
Aging analysis
Total past due	327,000		178,000
Commercial and industrial | 60 to 89 days past due
Aging analysis
Total past due	323,000		881,000
Commercial and industrial | 90 days or greater past due
Aging analysis
Total past due	1,121,000		154,000
Commercial real estate
Aging analysis
Total past due	1,216,000		1,762,000
Total current loans	853,297,000		793,971,000
Recorded Investment with Allowance	854,513,000		795,733,000
Commercial real estate | 30 to 59 days past due
Aging analysis
Total past due	479,000		68,000
Commercial real estate | 60 to 89 days past due
Aging analysis
Total past due	47,000		1,089,000
Commercial real estate | 90 days or greater past due
Aging analysis
Total past due	690,000		605,000
Construction and development
Aging analysis
Total past due	106,000		4,563,000
Total current loans	559,566,000		510,970,000
Recorded Investment with Allowance	559,672,000		515,533,000
Construction and development | 30 to 59 days past due
Aging analysis
Total past due	106,000		359,000
Construction and development | 60 to 89 days past due
Aging analysis
Total past due			4,204,000
1-4 family residential
Aging analysis
Total past due	297,000		542,000
Total current loans	281,228,000		281,469,000
Recorded Investment with Allowance	281,525,000		282,011,000
1-4 family residential | 30 to 59 days past due
Aging analysis
Total past due	254,000		395,000
1-4 family residential | 60 to 89 days past due
Aging analysis
Total past due	37,000		111,000
1-4 family residential | 90 days or greater past due
Aging analysis
Total past due	6,000		36,000
Multi‑family residential
Aging analysis
Total current loans	298,887,000		221,194,000
Recorded Investment with Allowance	298,887,000		221,194,000
Consumer
Aging analysis
Total past due			28,000
Total current loans	39,803,000		39,393,000
Recorded Investment with Allowance	39,803,000		39,421,000
Consumer | 30 to 59 days past due
Aging analysis
Total past due			28,000
Agriculture
Aging analysis
Total current loans	9,923,000		11,076,000
Recorded Investment with Allowance	9,923,000		11,076,000
Other
Aging analysis
Total past due	9,000
Total current loans	65,462,000		68,382,000
Recorded Investment with Allowance	65,471,000		$ 68,382,000
90 days past due and still accruing	9,000
Other | 90 days or greater past due
Aging analysis
Total past due	$ 9,000