CAPITAL STOCK - OPTION ACTIVITY (Details) - $ / shares	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 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)	2.45%	2.64%
Expected lives (in years)	4 years 8 months 6 days	4 years 10 months 24 days
Expected volatility (as a percent)	39.00%	40.00%
Expected dividend rate (as a percent)	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	1,200		153,700
Forfeited	0		(18,978)
Expired	0		(896)
Exercised	(122,895)		(144,120)
Outstanding at end of period	498,858		620,553
Exercisable at end of period	319,523		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)	98.970		75.244
Forfeited (in dollars per share)	0.000		53.010
Expired (in dollars per share)	0.000		25.740
Exercised (in dollars per share)	17.013		65.414
Outstanding at ending of period (in dollars per share)	46.727		40.741
Exercisable at end of period (in dollars per share)	$ 31.026		$ 21.214
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	2.35%	1.03%
Expected lives (in years)	1 year 1 month 6 days	1 year 2 months 18 days
Expected volatility (as a percent)	40.00%	44.00%
Expected dividend rate (as a percent)	0.00%	0.00%

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.