Segment Information - Classification of Segments (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2018
Segment Reporting Information [Line Items]
Operating Income			$ 49,188	$ 66,200	$ 55,445
Total Assets			872,686	560,871	527,029
Capital Expenditures			29,630	24,915	28,159
Depreciation and Amortization			32,731	25,939	19,401
Performance Materials Segment
Segment Reporting Information [Line Items]
Total Assets						$ 1,300
Operating Segments | Performance Materials Segment
Segment Reporting Information [Line Items]
Operating Income	$ 13,139	$ 12,321			12,574
Total Assets			414,211	72,837	66,965
Capital Expenditures			11,288	3,610	4,098
Depreciation and Amortization			9,006	3,996	4,028
Operating Segments | Technical Nonwovens Segment
Segment Reporting Information [Line Items]
Operating Income	21,323	26,047			15,584
Total Assets			242,007	271,713	268,104
Capital Expenditures			5,864	2,903	1,248
Depreciation and Amortization			13,877	12,625	6,791
Operating Segments | Thermal Acoustical Solutions Segment
Segment Reporting Information [Line Items]
Operating Income	38,085	53,132			53,072
Total Assets			201,509	189,301	166,591
Capital Expenditures			11,934	17,462	22,324
Depreciation and Amortization			9,190	8,619	7,605
Corporate
Segment Reporting Information [Line Items]
Operating Income	$ (23,359)	$ (25,300)			(25,785)
Total Assets			14,959	27,020	25,369
Capital Expenditures			544	940	489
Depreciation and Amortization			$ 658	$ 699	$ 977