SHARE-BASED COMPENSATION - RSUs to Employees (Details) - RSUs - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SHARE-BASED COMPENSATION
Shares granted to employees	1,105,640	706,637	759,702
Total compensation cost at the date of grant	$ 24,918,000	$ 12,179,000	$ 10,225,000
Total recognized compensation cost	12,350	$ 10,682	$ 10,258
Total unrecognized share-based compensation costs	$ 31,116
Weighted-average period of recognition of compensation expense	2 years 9 months 21 days
Number of Shares
Unvested at the beginning of the period (in shares)	1,659,767
Granted (in shares)	1,105,640	706,637	759,702
Vested (in shares)	(736,542)
Forfeited (in shares)	(140,112)
Unvested at the end of the period (in shares)	1,888,753	1,659,767
Weighted Average Grant-Date Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 15.26
Granted (in dollars per share)	22.80
Vested (in dollars per share)	14.85
Forfeited (in dollars per share)	16.07
Unvested at the end of the period (in dollars per share)	$ 19.78	$ 15.26
Total fair value of shares vested	$ 14,420	$ 10,733	$ 10,242
Minimum
SHARE-BASED COMPENSATION
Vesting period	1 year
Maximum
SHARE-BASED COMPENSATION
Vesting period	4 years

Total compensation cost of unvested share-based awards, other than options, awarded to employees as compensation.

No definition available.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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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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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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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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- Definition

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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- Definition

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

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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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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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