GUARANTOR SUBSIDIARIES FINANCIAL INFORMATION - Condensed Cash Flow Statement (FY) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	$ 38,038	$ 26,511	$ 188,811	$ 450,530	$ 450,844
Net cash (used in) provided by investing activities			(1,033,474)	(232,966)	(177,194)
Net cash provided by (used in) financing activities			(97,431)	522,971	(251,498)
Effect of changes in currency exchange rates	(27,208)	34,258	32,263	(26,436)	(18,868)
(Decrease) increase in cash			(909,831)	714,099	(3,284)
Cash, cash equivalents, and restricted cash, beginning of period	233,401	1,143,232	1,143,232	429,133	425,849
Cash, cash equivalents, and restricted cash, end of period	2,135,381	228,080	233,401	1,143,232	429,133
Elimination
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	(73,872)	(26,405)	(30,605)	(43,825)	(62,213)
Net cash (used in) provided by investing activities			0	0	0
Net cash provided by (used in) financing activities			30,605	43,825	62,213
Effect of changes in currency exchange rates	0	0	0	0	0
(Decrease) increase in cash			0	0	0
Cash, cash equivalents, and restricted cash, beginning of period	0	0	0	0	0
Cash, cash equivalents, and restricted cash, end of period	0	0	0	0	0
Parent
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	(111,120)	(38,959)	(49,231)	(44,611)	(90,374)
Net cash (used in) provided by investing activities			(11,156)	(829,783)	(7,862)
Net cash provided by (used in) financing activities			58,798	876,916	(48,570)
Effect of changes in currency exchange rates	0	0	0	0	0
(Decrease) increase in cash			(1,589)	2,522	(146,806)
Cash, cash equivalents, and restricted cash, beginning of period	933	2,522	2,522	0	146,806
Cash, cash equivalents, and restricted cash, end of period	1,760,805	3,049	933	2,522	0
Guarantors
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	95,503	101,597	130,298	142,839	300,965
Net cash (used in) provided by investing activities			(3,357)	(2,653)	(2,653)
Net cash provided by (used in) financing activities			(127,542)	(147,586)	(295,133)
Effect of changes in currency exchange rates	0	0	0	0	0
(Decrease) increase in cash			(601)	(7,400)	3,179
Cash, cash equivalents, and restricted cash, beginning of period	625	1,226	1,226	8,626	5,447
Cash, cash equivalents, and restricted cash, end of period	73	850	625	1,226	8,626
Non-Guarantors
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities	127,527	(9,722)	138,349	396,127	302,466
Net cash (used in) provided by investing activities			(1,018,961)	599,470	(166,679)
Net cash provided by (used in) financing activities			(59,292)	(250,184)	29,992
Effect of changes in currency exchange rates	(27,208)	34,258	32,263	(26,436)	(18,868)
(Decrease) increase in cash			(907,641)	718,977	(146,911)
Cash, cash equivalents, and restricted cash, beginning of period	231,843	1,139,484	1,139,484	420,507	273,596
Cash, cash equivalents, and restricted cash, end of period	$ 374,503	$ 224,181	$ 231,843	$ 1,139,484	$ 420,507

Amount of increase (decrease) in cash and cash equivalents. Cash and cash equivalents are the amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Includes effect from exchange rate changes.

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Amount of cash and cash equivalents, and cash and cash equivalents restricted to withdrawal or usage. Excludes amount for disposal group and discontinued operations. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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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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Amount of increase (decrease) from the effect of exchange rate changes on cash and cash equivalent balances held in foreign currencies.

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Amount of cash inflow (outflow) of financing activities, excluding discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Amount of cash inflow (outflow) of investing activities, excluding discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

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Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

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