CAPITAL STOCK - OPTION ACTIVITY (Details) - $ / shares	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	1.76%	1.41%	1.21%
Expected lives (in years)	4 years 5 months 24 days	3 years 4 months 24 days	3 years 4 months 21 days
Expected volatility (as a percent)	41.00%	41.00%	43.00%
Expected dividend rate (as a percent)	0.00%	0.00%	0.00%
Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period	940,610	1,074,251	1,321,232
Granted at Market	129,855	146,446	134,800
Canceled	(463)	(28,440)	(218,926)
Exercised	(240,385)	(251,647)	(162,855)
Outstanding at end of period	829,617	940,610	1,074,251
Exercisable at end of period	532,703	621,559	729,175
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)	$ 14.163	$ 10.110	$ 10.386
Granted at Market (in dollars per share)	67.706	36.904	16.398
Cancelled (in dollars per share)	14.881	10.080	17.786
Exercised (in dollars per share)	11.886	10.559	7.234
Outstanding at ending of period (in dollars per share)	23.203	14.163	10.110
Exercisable at end of period (in dollars per share)	$ 12.140	$ 10.269	$ 9.800
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	0.54%	0.27%	0.23%
Expected lives (in years)	1 year 2 months 18 days	1 year 2 months 18 days	1 year 3 months 18 days
Expected volatility (as a percent)	42.00%	36.00%	34.00%
Expected dividend rate (as a percent)	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.