STOCKHOLDERS' EQUITY - Restricted Stock and Performance Share Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Unrecognized compensation cost related to non-vested shares	$ 8,565	$ 7,742	$ 8,260
Weighted-average period of recognition for unrecognized compensation cost	1 year 7 months 6 days
Estimated share-based compensation expense for current fiscal year	$ 7,700
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested balance as of beginning of year (in shares)	66	102	150
Granted (in shares)	42	21	19
Vested (in shares)	(27)	(53)	(66)
Forfeited (in shares)	(2)	(4)	(1)
Non-vested balance as of end of year (in shares)	79	66	102
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Non-vested balance as of beginning of year (in dollars per share)	$ 65.66	$ 54.18	$ 47.46
Granted (in dollars per share)	77.50	83.43	61.22
Vested (in dollars per share)	62.74	51.39	40.96
Forfeited (in dollars per share)	74.57	55.45	56.77
Non-vested balance as of end of year (in dollars per share)	$ 72.75	$ 65.66	$ 54.18
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested balance as of beginning of year (in shares)	39	34	20
Granted (in shares)	32	16	22
Vested (in shares)	(15)	0	0
Forfeited (in shares)	(3)	(11)	(8)
Non-vested balance as of end of year (in shares)	53	39	34
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Non-vested balance as of beginning of year (in dollars per share)	$ 72.62	$ 61.06	$ 58.77
Granted (in dollars per share)	71.27	93.85	63.15
Vested (in dollars per share)	58.78	0	0
Forfeited (in dollars per share)	72.55	69.25	60.88
Non-vested balance as of end of year (in dollars per share)	$ 75.61	$ 72.62	$ 61.06

Estimated share-based compensation expense for current fiscal year.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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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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- 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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- Definition

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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- Definition

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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- 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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