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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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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.

CAPITAL STOCK - OPTION ACTIVITY (Details) - $ / shares	3 Months Ended		9 Months Ended		12 Months Ended
CAPITAL STOCK - OPTION ACTIVITY (Details) - $ / shares	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2017
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	2.80%	1.83%	2.66%	1.75%
Expected lives (in years)	4 years 10 months 18 days	4 years 10 months 24 days	4 years 10 months 18 days	4 years 10 months 24 days
Expected volatility (as a percent)	40.00%	39.00%	40.00%	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			630,847	829,617
Granted at market			152,200		27,050
Canceled			(19,708)		(18,331)
Exercised			(98,594)		(207,489)
Outstanding at end of period	664,745		664,745		829,617
Exercisable at end of period	414,018		414,018			456,802
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)			$ 29.312	$ 23.203
Granted at market (in dollars per share)			75.010		$ 99.087
Cancelled (in dollars per share)			53.109		57.197
Exercised (in dollars per share)			30.639		11.520
Outstanding at ending of period (in dollars per share)	$ 38.873		38.873		$ 23.203
Exercisable at end of period (in dollars per share)	$ 19.378		$ 19.378			$ 18.316
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	2.04%	0.76%	1.39%	0.70%
Expected lives (in years)	1 year 1 month 23 days	1 year 2 months 18 days	1 year 2 months 18 days	1 year 2 months 18 days
Expected volatility (as a percent)	42.00%	45.00%	43.00%	45.00%
Expected dividend rate (as a percent)	0.00%	0.00%	0.00%	0.00%