STOCK-BASED COMPENSATION - Equity Instruments Other Than Options (Details) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested, January 1, 2018	110,683
Number of Shares, Granted	51,968
Number of Shares, Vested	(69,670)
Number of Shares, Forfeited	(600)
Number of Shares, Unvested, December 31, 2018	92,381	110,683
Weighted-Average Grant-Date Fair Value, Unvested, January 1, 2018	$ 88.91
Weighted-Average Grant-Date Fair Value, Granted	115.48	$ 116.58	$ 53.85
Weighted-Average Grant-Date Fair Value, Vested	97.14
Weighted-Average Grant-Date Fair Value, Forfeited	103.09
Weighted-Average Grant-Date Fair Value, Unvested, December 31, 2018	$ 97.56	$ 88.91
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested, January 1, 2018	579,937
Number of Shares, Granted	2,456
Number of Shares, Vested	(172,444)
Number of Shares, Unvested, December 31, 2018	409,949	579,937
Weighted-Average Grant-Date Fair Value, Unvested, January 1, 2018	$ 46.72
Weighted-Average Grant-Date Fair Value, Granted	122.15	$ 83.25	$ 65.37
Weighted-Average Grant-Date Fair Value, Vested	47.75
Weighted-Average Grant-Date Fair Value, Unvested, December 31, 2018	$ 46.74	$ 46.72
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested, January 1, 2018	73,315
Number of Shares, Granted	40,601	24,664	25,045
Number of Shares, Vested	(61,186)
Number of Shares, Unvested, December 31, 2018	52,730	73,315
Weighted-Average Grant-Date Fair Value, Unvested, January 1, 2018	$ 62.36
Weighted-Average Grant-Date Fair Value, Granted	119.62	$ 105.65	$ 58.46
Weighted-Average Grant-Date Fair Value, Vested	45.19
Weighted-Average Grant-Date Fair Value, Unvested, December 31, 2018	$ 86.43	$ 62.36

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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Per share weighted average intrinsic value of equity-based compensation awards forfeited. Excludes stock and unit options.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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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).

+ References

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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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Per share weighted average intrinsic value of equity-based compensation awards not vested. Excludes stock and unit options.

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-Publisher FASB
-Name Accounting Standards Codification
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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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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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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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