Guarantor Subsidiaries Financial Information - Condensed Cash Flow Statement (Q3) (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 for) provided by operating activities	$ 38,038	$ 26,511	$ 188,811	$ 450,530	$ 450,844
Net cash used for investing activities	(104,160)	(905,872)	(1,033,474)	(232,966)	(177,194)
Net cash provided by (used for) financing activities	1,995,310	(70,049)	(97,431)	522,971	(251,498)
Effect of changes in currency exchange rates	(27,208)	34,258	32,263	(26,436)	(18,868)
Increase (decrease) in cash	1,901,980	(915,152)
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 for) provided by operating activities	(73,872)	(26,405)	(30,605)	(43,825)	(62,213)
Net cash used for investing activities	0	0
Net cash provided by (used for) financing activities	73,872	26,405
Effect of changes in currency exchange rates	0	0	0	0	0
Increase (decrease) in cash	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 for) provided by operating activities	(111,120)	(38,959)	(49,231)	(44,611)	(90,374)
Net cash used for investing activities	(12,498)	(12,591)
Net cash provided by (used for) financing activities	1,883,490	52,077
Effect of changes in currency exchange rates	0	0	0	0	0
Increase (decrease) in cash	1,759,872	527
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 for) provided by operating activities	95,503	101,597	130,298	142,839	300,965
Net cash used for investing activities	(1,303)	(2,358)
Net cash provided by (used for) financing activities	(94,752)	(99,615)
Effect of changes in currency exchange rates	0	0	0	0	0
Increase (decrease) in cash	(552)	(376)
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 for) provided by operating activities	127,527	(9,722)	138,349	396,127	302,466
Net cash used for investing activities	(90,359)	(890,923)
Net cash provided by (used for) financing activities	132,700	(48,916)
Effect of changes in currency exchange rates	(27,208)	34,258	32,263	(26,436)	(18,868)
Increase (decrease) in cash	142,660	(915,303)
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 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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 24
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3521-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Section 50
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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) from financing activities, including 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) from investing activities, including 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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