Stock-Based Compensation, Assumptions Used to Value Stock Options Granted (Q3) (Details) - $ / shares	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumptions Used in Determining Fair Value of Stock Options Granted [Abstract]
Volatility	0.00%	86.98%	89.06%	88.82%	88.87%	77.20%	79.80%
Risk-free interest rate	0.00%	1.83%	2.31%	1.89%	1.88%	1.39%	1.74%
Expected term	0 years	6 years 11 days	4 years 2 months 26 days	5 years 11 months 26 days	6 years	6 years 7 days	6 years 18 days
Dividend rate	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Fair value of option on grant date (in dollars per share)	$ 0.00	$ 7.22	$ 5.54	$ 6.76	$ 6.93	$ 5.39	$ 5.42

No definition available.

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- Definition

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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- Definition

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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- Definition

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

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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 (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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