FAIR VALUE OF FINANCIAL INSTRUMENTS - Carrying Value (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Cost-method Investments Line Items
Original Cost		$ 11,205	$ 9,955
Other Than Temporary Impairment	$ 0	0
Carrying Amount Not Evaluated for Impairment		11,205	$ 9,955
Payments to Acquire Restricted Investments	0	1,250
Cost Method Invetsment, after adjustments	11,205	11,205
Senior Notes
Fair Value Of Instruments [Line Items]
Carrying value of debt	400,000	400,000
Fair value of debt	399,500	378,000
Finance Method Lease Obligations
Fair Value Of Instruments [Line Items]
Carrying value of debt	889	912
Letter of credit
Fair Value Of Instruments [Line Items]
Carrying value of debt	5,862	5,862
New Term B Loan [Member]
Fair Value Of Instruments [Line Items]
Carrying value of debt	1,291,700	1,291,700
Fair value of debt	1,259,408	1,243,261
New Revolver [Member]
Fair Value Of Instruments [Line Items]
Carrying value of debt	0	180,000
Fair value of debt	$ 0	$ 180,000

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Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

+ References

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This element represents the aggregate carrying amount of all cost-method investments held by the Entity which was not evaluated for impairment. Because the fair value of cost-method investments is not readily determinable, the evaluation of whether an investment is impaired generally consists of whether an event or change in circumstances has occurred during the reporting period that may have a significant adverse effect on the fair value of the investment (an "impairment indicator"). Impairment indicators include, but are not limited to: (1) significant deterioration in the earnings performance, credit rating, asset quality, or business prospects of the investee; (2) significant adverse change in the regulatory, economic, or technological environment of the investee; (3) significant adverse change in the general market condition of either the geographic area or the industry in which the investee operates; (4) a bona fide offer to purchase (whether solicited or unsolicited), an offer by the investee to sell, or a completed auction process for the same or similar security for an amount less than the cost of the investment; or (5) factors that raise significant concerns about the investee's ability to continue as a going concern, such as negative cash flows from operations, working capital deficiencies, or noncompliance with statutory capital requirements or debt covenants.

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Original amount of cost-method investments at the acquisition date.

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the cost method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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The cash outflow to acquire investments (not to include restricted cash) that are pledged or subject to withdrawal restrictions.

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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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