STOCKHOLDERS' EQUITY - Stock Options (Details) - Stock Options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares)	887	946
Granted (in shares)	185	148
Exercised (in shares)	(7)	(39)
Forfeited (in shares)	0	(1)
Canceled (in shares)	0	(1)
Outstanding at end of period (in shares)	1,065	1,053
Exercisable at end of period (in shares)	685	637
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding at beginning of period (in dollars per share)	$ 61.59	$ 55.44
Granted (in dollars per share)	84.19	74.57
Exercised (in dollars per share)	40.85	32.83
Forfeited (in dollars per share)	0	73.49
Canceled (in dollars per share)	0	25.39
Outstanding at end of period (in dollars per share)	65.64	58.98
Exercisable at end of period	$ 56.91	$ 48.54
Aggregate intrinsic value, outstanding, beginning of period	$ 16,192	$ 24,714
Aggregate intrinsic value, outstanding, end of period	28,924	24,680
Aggregate intrinsic value, exercisable, end of period	$ 24,588	$ 21,306
Weighted average remaining contractual term, outstanding	6 years 8 months 12 days	6 years 7 months 6 days
Weighted average remaining contractual term, exercisable	5 years 4 months 24 days	5 years 3 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield rate	0.60%	0.60%
Expected volatility rate	24.00%	27.00%
Risk-free interest rate	2.50%	2.60%
Expected term	4 years	4 years 4 months 24 days
Weighted-average fair value of options granted	$ 18.26	$ 18.62
Total intrinsic value of stock options exercised	$ 302	$ 1,825

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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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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.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Net number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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