Industry Segments and Foreign Operations - Sales Revenue by Product Groups (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenue	$ 456,411	$ 502,392
Cost of goods sold	349,409	355,314
Gross profit	107,002	147,078
Operating expenses:
Research and development	10,551	10,797
Selling, general, and administrative	40,894	38,723
Depreciation and amortization	31,522	35,376
Gain on insurance proceeds	(11,100)	0
(Gain) loss on disposal of fixed assets	0	27
Operating income	35,135	62,155
Other expense	(259)	(1,113)
Gain (loss) on extinguishment of debt	210	(7,591)
Earnings of unconsolidated joint venture	121	137
Interest expense, net	(18,941)	(29,276)
Income before income taxes	16,266	24,312
Polymer
Segment Reporting Information [Line Items]
Revenue	261,055	289,071
Cost of goods sold	207,169	207,640
Gross profit	53,886	81,431
Operating expenses:
Research and development	7,567	7,447
Selling, general, and administrative	23,098	23,520
Depreciation and amortization	13,971	17,762
Gain on insurance proceeds	0	0
(Gain) loss on disposal of fixed assets	0	(98)
Operating income	9,250	32,800
Chemical
Segment Reporting Information [Line Items]
Revenue	195,356	213,321
Cost of goods sold	142,240	147,674
Gross profit	53,116	65,647
Operating expenses:
Research and development	2,984	3,350
Selling, general, and administrative	17,796	15,203
Depreciation and amortization	17,551	17,614
Gain on insurance proceeds	(11,100)	0
(Gain) loss on disposal of fixed assets	0	125
Operating income	$ 25,885	$ 29,355