STOCK PLANS AND STOCK BASED COMPENSATION - Activity in Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Shares
Outstanding, beginning of year (in shares)		193,114
Granted (in shares)		0	0
Exercised (in shares)		(86,556)
Forfeited or expired (in shares)		(400)
Outstanding at end of period (in shares)		106,158		193,114
Exercisable end of period (in shares)		94,754
Weighted Average Exercise Price
Outstanding, beginning of year (in dollars per share)		$ 12.09
Granted (in dollars per share)	$ 34.15
Exercised (in dollars per share)		12.59
Forfeited or expired (in dollars per share)		13.67
Outstanding, period end (in dollars per share)		11.67		$ 12.09
Exercisable at end of period (in dollars per share)		$ 11.13
Stock options vested and currently exercisable
Outstanding at end of period, Weighted average remaining contractual term (years)		4 years 4 months 24 days
Outstanding at end of period, Aggregate intrinsic value		$ 2,568
Exercisable at end of period, Weighted average remaining contractual term (years)		4 years 1 month 6 days
Exercisable at end of period, Aggregate intrinsic value		$ 2,344
Stock options
Shares
Exercisable end of period (in shares)		94,754		181,710
Weighted Average Exercise Price
Exercisable at end of period (in dollars per share)		$ 11.13		$ 11.83
Stock options vested and currently exercisable
Exercisable at end of period, Weighted average remaining contractual term (years)		4 years 1 month 6 days		3 years 9 months 18 days
Exercisable at end of period, Aggregate intrinsic value		$ 2,344		$ 4,101