STOCK-BASED COMPENSATION - Summary of RSA activity (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
STOCK-BASED COMPENSATION
Unvested, Shares, Outstanding, Beginning of period	117	85	63
Unvested Shares, Granted	122	65	50
Unvested Shares, Vested	(42)	(33)	(28)
Unvested Shares, Forfeited	(5)	0	0
Unvested Shares, Outstanding, End of period	192	117	85	63
Weighted Average Grant Date Fair Value, Outstanding, Beginning of period	$ 54.04	$ 48.34	$ 45.72
Weighted Average Grant Date Fair Value, Granted	66.39	58.11	49.51
Weighted Average Grant Date Fair Value, Vested	54.77	47.34	44.49
Weighted Average Grant Date Fair Value, Forfeited	62.63	0.00	0.00
Weighted Average Grant Date Fair Value, Outstanding, End of period	$ 61.46	$ 54.04	$ 48.34	$ 45.72
Weighted Average Remaining Term (years)	2 years 7 months 6 days	2 years 1 month 6 days	2 years 7 months 6 days	2 years 2 months 12 days