SHARE-BASED COMPENSATION - RSUs to Employees (Details) - RSUs - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SHARE-BASED COMPENSATION
Shares granted to employees	759,702	1,033,001	868,036
Total compensation cost at the date of grant	$ 10,200	$ 13,300	$ 12,100
Total recognized compensation cost	10,227	$ 9,314	$ 7,713
Total unrecognized share-based compensation costs	$ 20,452
Weighted-average period of recognition of compensation expense	2 years 5 months 1 day
Number of Shares
Unvested at the beginning of the period (in shares)	1,732,047
Granted (in shares)	759,702	1,033,001	868,036
Vested (in shares)	(609,418)
Forfeited (in shares)	(101,060)
Unvested at the end of the period (in shares)	1,781,271	1,732,047
Weighted Average Grant-Date Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 15.42
Granted (in dollars per share)	13.62
Vested (in dollars per share)	16.84
Forfeited (in dollars per share)	15.17
Unvested at the end of the period (in dollars per share)	$ 14.18	$ 15.42
Total fair value of shares vested	$ 10,242	$ 12,091	$ 7,271
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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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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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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- Definition

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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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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- Definition

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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- Definition

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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