INVESTMENT SECURITIES - Other investment disclosures (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Weighted average remaining maturity available for sale	11 years 4 months 24 days	7 years 8 months 12 days
Available for sale securities callable before maturity	$ 274,500,000	$ 239,000,000.0
Weighted average remaining maturity held to maturity	9.0 years	10.0 years
Shareholders' equity	$ 868,008,000	$ 819,018,000	$ 753,954,000		$ 785,944,000
Qualified affordable housing project investments	75,700,000	65,500,000
Affordable housing project investments, commitment	35,400,000	27,800,000
Affordable housing project impairment	0	0
Income Tax Expense
Schedule of Investments [Line Items]
Affordable housing project investments, amortization	4,900,000	4,300,000	3,000,000.0
Affordable housing tax credits and other tax benefits	5,000,000.0	4,100,000	2,900,000
Accumulated Other Comprehensive Income/ (Loss)
Schedule of Investments [Line Items]
Shareholders' equity	31,685,000	7,575,000	(17,719,000)		(8,658,000)
Retained Earnings
Schedule of Investments [Line Items]
Shareholders' equity	$ 156,431,000	$ 127,469,000	$ 87,821,000		$ 49,526,000
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income/ (Loss)
Schedule of Investments [Line Items]
Shareholders' equity				$ (339,000)
Accounting Standards Update 2016-01 | Retained Earnings
Schedule of Investments [Line Items]
Shareholders' equity				$ 339,000