CAPITAL STOCK - OPTION ACTIVITY (Details) - $ / shares	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	1.83%	1.14%	1.75%	1.19%
Expected lives (in years)	4 years 10 months 18 days	4 years 5 months 18 days	4 years 10 months 18 days	4 years 5 months 18 days
Expected volatility (as a percent)	39.00%	40.00%	41.00%	41.00%
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			829,617	940,610	940,610
Granted at Market			26,550		129,855
Canceled			(3,895)		(463)
Exercised			(189,350)		(240,385)
Outstanding at end of period	662,922		662,922		829,617
Exercisable at end of period	445,542		445,542		532,703
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)			$ 23.203	$ 14.163	$ 14.163
Granted at Market (in dollars per share)			99.127		67.706
Cancelled (in dollars per share)			45.635		14.881
Exercised (in dollars per share)			10.877		11.886
Outstanding at ending of period (in dollars per share)	$ 29.633		29.633		23.203
Exercisable at end of period (in dollars per share)	$ 17.075		$ 17.075		$ 12.140
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	0.76%	0.57%	0.70%	0.54%
Expected lives (in years)	1 year 2 months 18 days	1 year 2 months 18 days	1 year 2 months 18 days	1 year 2 months 18 days
Expected volatility (as a percent)	45.00%	44.00%	45.00%	43.00%
Expected dividend rate (as a percent)	0.00%	0.00%	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.