STOCK-BASED COMPENSATION - Combined Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) $ / shares shares
Shares
Options outstanding, beginning of period (shares) \| shares	7,130
Granted (shares) \| shares	1,705
Exercised (shares) \| shares	(1,422)
Forfeited (shares) \| shares	(442)
Options outstanding, end of period (shares) \| shares	6,971
Vested and expected to vest (shares) \| shares	5,841
Exercisable (shares) \| shares	2,893
Weighted-average Exercise Price
Options outstanding, beginning of period (dollars per share) \| $ / shares	$ 26.10
Granted (dollars per share) \| $ / shares	58.42
Exercised (dollars per share) \| $ / shares	20.88
Forfeited (dollars per share) \| $ / shares	35.50
Options outstanding, end of period (dollars per share) \| $ / shares	34.47
Vested and expected to vest (dollars per share) \| $ / shares	33.23
Exercisable (dollars per share) \| $ / shares	$ 22.16
Weighted-average remaining contractual term
Vested and expected to vest	7 years
Exercisable	5 years 8 months 12 days
Aggregate intrinsic value
Vested and expected to vest \| $	$ 99,999
Exercisable \| $	$ 74,650

Share-based Compensation Arrangement by Share-based Payment Award, Options, Aggregate Intrinsic Value [Abstract]

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Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Remaining Contractual Term [Abstract]

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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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For presentations that combine terminations, 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 or that expired.

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Weighted average price of options that were either forfeited or expired.

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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.

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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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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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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 fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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