CAPITAL STOCK - OPTION ACTIVITY (Details) - $ / shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	1.89%	2.76%	1.95%	2.64%
Expected lives (in years)	4 years 8 months 6 days	4 years 10 months 24 days	4 years 8 months 6 days	4 years 10 months 24 days
Expected volatility (as a percent)	40.00%	40.00%	40.00%	40.00%
Expected dividend rate (as a percent)	0.00%	0.00%	0.00%	0.00%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period			620,553	630,847	630,847
Granted at market			11,200		153,700
Forfeited			(256)		(18,978)
Expired			0		(896)
Exercised			(149,828)		(144,120)
Outstanding at end of period	481,669		481,669		620,553
Exercisable at end of period	305,452		305,452		386,176
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding at beginning of period (in dollars per share)			$ 40.741	$ 29.312	$ 29.312
Granted at market (in dollars per share)			73.193		75.244
Forfeited (in dollars per share)			98.660		53.010
Expired (in dollars per share)			0.000		65.414
Exercised (in dollars per share)			16.776		25.740
Outstanding at ending of period (in dollars per share)	$ 48.920		48.920		40.741
Exercisable at end of period (in dollars per share)	$ 33.655		$ 33.655		$ 21.214
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	2.21%	1.09%	2.27%	1.06%
Expected lives (in years)	1 year 1 month 6 days	1 year 2 months 18 days	1 year 1 month 6 days	1 year 2 months 18 days
Expected volatility (as a percent)	39.00%	44.00%	39.00%	44.00%
Expected dividend rate (as a percent)	0.00%	0.00%	0.00%	0.00%