STOCKHOLDERS' EQUITY AND COMPENSATION ARRANGEMENTS - RSU (Details) - Restricted stock units - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
Sep. 30, 2021
Number of Shares
Beginning balance (in shares)	1,853,007
Granted (in shares)	936,875
Vested (in shares)	(1,002,467)
Forfeited (in shares)	(286,316)
Ending balance (in shares)	1,501,099
Weighted-Average Grant Date Fair Value (per unit)
Beginning balance (in dollars per share)	$ 31.91
Granted (in dollars per share)	42.97
Vested (in dollars per share)	29.34
Forfeited (in dollars per share)	37.54
Ending balance (in dollars per share)	$ 39.28
Unrecognized compensation costs	$ 41.9
Weighted-average period of recognition	11 months 23 days
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	1 year
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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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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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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