STOCK PLANS - Stock Option Activity (Details) - Stock options - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Options
Outstanding at beginning of year (in shares)	67,500	270,000	647,671
Exercised (in shares)	(22,500)	(202,500)	(377,671)
Expired (in shares)	(45,000)
Outstanding at end of year (in shares)	0	67,500	270,000	647,671
Exercisable at end of year (in shares)	0
Weighted-Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 22.64	$ 15.93	$ 15.37
Exercised (in dollars per share)	20.97	13.69	14.98
Expired (in dollars per share)	23.47
Outstanding at end of year (in dollars per share)	0.00	$ 22.64	$ 15.93	$ 15.37
Exercisable at end of year (in dollars per share)	$ 0.00
Weighted- Average Remaining Contractual Life (years)
Outstanding at end of year		8 months 5 days	1 year 5 months 9 days	2 years 1 month 28 days
Exercisable at end of year
Aggregate Intrinsic Value
Outstanding at beginning of year	$ 357,225	$ 1,203,600	$ 4,016,809
Exercised	147,760	1,597,398	2,496,218
Outstanding at end of year	0	$ 357,225	$ 1,203,600	$ 4,016,809
Exercisable at end of year	$ 0