Industry Segments and Foreign Operations - Long-lived Assets Including Goodwill and Total Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Property, plant, and equipment, net	$ 942,834	$ 906,722
Investment in unconsolidated joint venture	12,098	11,195
Goodwill	773,635	770,012
Total assets	2,992,606	2,906,645
Polymer
Segment Reporting Information [Line Items]
Property, plant, and equipment, net	559,170	548,994
Investment in unconsolidated joint venture	12,098	11,195
Goodwill	0	0
Total assets	1,168,577	1,127,273
Chemical
Segment Reporting Information [Line Items]
Property, plant, and equipment, net	383,664	357,728
Investment in unconsolidated joint venture	0	0
Goodwill	773,635	770,012
Total assets	$ 1,824,029	$ 1,779,372