STOCKHOLDERS' EQUITY - Restricted Stock and Performance Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Unrecognized compensation cost	$ 15,475	$ 11,860
Period for unrecognized compensation cost to be recognized over	1 year 7 months 6 days
Estimated share-based compensation expense	$ 7,600
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 period (in shares)	79	66
Granted (in shares)	67	37
Vested (in shares)	(8)	(17)
Forfeited (in shares)	(1)	0
Non-vested balance as of end of period (in shares)	137	86
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Non-vested balance as of beginning of period (in dollars per share)	$ 72.75	$ 65.66
Granted (in dollars per share)	84.44	75.19
Vested (in dollars per share)	58.52	57.65
Forfeited (in dollars per share)	76.73	0
Non-vested balance as of end of period (in dollars per share)	$ 79.31	$ 71.40
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 period (in shares)	53	39
Granted (in shares)	33	32
Vested (in shares)	(9)	(15)
Forfeited (in shares)	(7)	0
Non-vested balance as of end of period (in shares)	70	56
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Non-vested balance as of beginning of period (in dollars per share)	$ 75.61	$ 72.62
Granted (in dollars per share)	81.79	71.27
Vested (in dollars per share)	65.64	58.78
Forfeited (in dollars per share)	60.85	0
Non-vested balance as of end of period (in dollars per share)	$ 81.26	$ 75.47
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate	2.50%	2.40%
Dividend yield rate	0.50%	0.50%
Expected volatility rate	24.00%	27.00%
Initial TSR	(5.90%)	(10.50%)
Cliff vest	100.00%

Estimated share-based compensation expense for current fiscal year.

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The initial percentage of total shareholder return.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

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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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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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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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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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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Percentage of vesting of share-based compensation awards.

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