STOCK-BASED COMPENSATION - SUMMARY OF THE COMPANY'S OPTION ACTIVITY (Details) - Employee Stock Option	6 Months Ended
Jun. 30, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding as of beginning of period (in shares)	5,557,561
Granted (in shares)	0
Exercised (in shares)	(497,008)
Forfeited (in shares)	(6,872)
Expired (in shares)	0
Outstanding as of end of period (in shares)	5,053,681

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(4)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

+ References

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

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

+ References

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

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