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

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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 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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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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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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Number of shares authorized for issuance under share-based payment arrangement.

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SHARE-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based compensation
Number of shares authorized	4,300,000
Restricted Stock Units
Award activity
Outstanding at the beginning of the period (in shares)	1,617,120
Granted (in shares)	579,660	386,840	231,510
Vested (in shares)	(320,788)
Forfeited (in shares)	(34,842)
Outstanding at the end of the period (in shares)	1,841,150	1,617,120
Weighted-Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 24.82
Granted (in dollars per share)	19.22	$ 27.75	$ 44.50
Vested (in dollars per share)	30.29
Forfeited (in dollars per share)	25.48
Outstanding at the end of the period (in dollars per share)	$ 22.09	$ 24.82
Other disclosure
Fair value of restricted stock awards vested	$ 7.8	$ 4.9	$ 9.6
Unrecognized compensation cost	$ 18.0
Weighted-average period of recognition of unrecognized compensation cost	2 years
Restricted Stock Units \| Employee
Other disclosure
Vesting period	4 years	4 years	4 years	5 years