Industry Segments and Foreign Operations - Sales Revenue by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	$ 456,411	$ 502,392
United States
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	168,325	185,053
Germany
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	40,892	48,078
All other countries
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	247,194	269,261
Polymer
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	261,055	289,071
Polymer | United States
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	87,968	103,729
Polymer | Germany
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	25,911	31,842
Polymer | All other countries
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	147,176	153,500
Chemical
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	195,356	213,321
Chemical | United States
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	80,357	81,324
Chemical | Germany
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	14,981	16,236
Chemical | All other countries
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	$ 100,018	$ 115,761