STOCK-BASED COMPENSATION - Outstanding and Unvested RSUs and PSUs (Details) shares in Thousands	12 Months Ended
Dec. 31, 2018 $ / shares shares
RSUs
Number of Shares
Balance at beginning of the period (shares) \| shares	360
Granted (shares) \| shares	153
Vested (shares) \| shares	(49)
Forfeited (shares) \| shares	(5)
Balance at end of the period (shares) \| shares	459
Weighted Average Grant Date Fair Value
Balance at beginning of the period (USD per share) \| $ / shares	$ 80.81
Granted (USD per share) \| $ / shares	183.33
Vested (USD per share) \| $ / shares	78.54
Forfeited (USD per share) \| $ / shares	98.81
Balance at end of the period (USD per share) \| $ / shares	$ 115.12
Vesting period	3 years
PSUs
Number of Shares
Balance at beginning of the period (shares) \| shares	130
Granted (shares) \| shares	30
Vested (shares) \| shares	0
Forfeited (shares) \| shares	(17)
Balance at end of the period (shares) \| shares	143
Weighted Average Grant Date Fair Value
Balance at beginning of the period (USD per share) \| $ / shares	$ 76.00
Granted (USD per share) \| $ / shares	152.53
Vested (USD per share) \| $ / shares	0.00
Forfeited (USD per share) \| $ / shares	76.00
Balance at end of the period (USD per share) \| $ / shares	$ 92.02

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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- Definition

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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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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