STOCK-BASED COMPENSATION - Stock option activity (Details) - Stock options - $ / shares	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Number of Shares Underlying Options
Outstanding, beginning of period	213,078	232,322
Exercised	(1,524)	(10,844)
Outstanding, end of period	211,554	221,478
Weighted Average Exercise Price
Outstanding, beginning of period	$ 16.92	$ 16.66
Exercised	10.34	10.68
Outstanding, end of period	$ 16.97	$ 16.96

Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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