Goodwill and Long-Lived Assets - Changes in Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Beginning Balance	$ 68,969	$ 68,969	$ 63,606
Goodwill addition		131,509	323
Currency translation adjustment		(3,515)	5,040
Ending Balance		196,963	68,969
Performance Materials
Goodwill [Roll Forward]
Beginning Balance	13,307	13,307	12,777
Goodwill addition	131,500	131,509	0
Currency translation adjustment		(190)	530
Ending Balance		144,626	13,307
Technical Nonwovens
Goodwill [Roll Forward]
Beginning Balance	$ 55,662	55,662	50,829
Goodwill addition		0	323
Currency translation adjustment		(3,325)	4,510
Ending Balance		$ 52,337	$ 55,662

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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Amount of increase (decrease), classified as other, of an asset representing the future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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