Industry Segments and Foreign Operations - Long-lived Assets Including Goodwill and Total Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Property, plant, and equipment, net	$ 937,301	$ 941,476
Investment in unconsolidated joint venture	11,500	12,070
Goodwill	772,462	772,886
Total assets	2,956,938	2,894,704
Polymer
Segment Reporting Information [Line Items]
Property, plant, and equipment, net	541,194	543,086
Investment in unconsolidated joint venture	11,500	12,070
Goodwill	0	0
Total assets	1,155,949	1,160,029
Chemical
Segment Reporting Information [Line Items]
Property, plant, and equipment, net	396,107	398,390
Investment in unconsolidated joint venture	0	0
Goodwill	772,462	772,886
Total assets	$ 1,800,989	$ 1,734,675