Condensed consolidating financial information - Statement of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue, Net [Abstract]
Related parties	$ 299,235	$ 301,111	$ 876,079	$ 871,771
Other customers	101,410	32,539	279,154	107,487
Total net sales	400,645	333,650	1,155,233	979,258
Cost of goods sold	359,243	351,262	1,074,520	995,357
Gross profit (loss)	41,402	(17,612)	80,713	(16,099)
Selling, general and administrative expenses	13,994	9,733	34,048	29,303
Ravenswood (gains) losses	(5,500)	26,830	(5,500)	26,830
Other operating expense - net	437	878	1,590	2,337
Operating income (loss)	32,471	(55,053)	50,575	(74,569)
Interest expense	(5,542)	(5,531)	(16,574)	(16,521)
Intercompany interest	0	0	0	0
Interest income	422	190	935	475
Net (loss) on forward and derivative contracts	(3,888)	1,275	(17,068)	2,998
Other income (expense) - net	427	(157)	(1,065)	(462)
Income (loss) before income taxes and equity in earnings of joint ventures	23,890	(59,276)	16,803	(88,079)
Income tax (expense) benefit	(3,321)	848	(4,465)	3,237
Income (loss) before equity in earnings of joint ventures	20,569	(58,428)	12,338	(84,842)
Equity in earnings (loss) of joint ventures	214	155	437	891
Net income (loss)	20,783	(58,273)	12,775	(83,951)
Other comprehensive income (loss) before income tax effect	1,429	1,206	3,073	3,618
Income tax effect	(382)	(382)	(1,145)	(1,147)
Other comprehensive income	1,047	824	1,928	2,471
Total comprehensive income (loss)	21,830	(57,449)	14,703	(81,480)
Consolidating Adjustments [Member]
Revenue, Net [Abstract]
Related parties	0	(22,285)	0	(69,596)
Other customers	0	(267)	0	(267)
Total net sales	0	(22,552)	0	(69,863)
Cost of goods sold	0	(21,777)	0	(69,552)
Gross profit (loss)	0	(775)	0	(311)
Selling, general and administrative expenses	0	0	0	0
Ravenswood (gains) losses	0	0	0	0
Other operating expense - net	0	0	0	0
Operating income (loss)	0	(775)	0	(311)
Interest expense	0	0	0	0
Intercompany interest	0	0		0
Interest income	0	0	0	0
Net (loss) on forward and derivative contracts	0	0	0	0
Other income (expense) - net	0	0	0	0
Income (loss) before income taxes and equity in earnings of joint ventures	0	(775)	0	(311)
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in earnings of joint ventures	0	(775)	0	(311)
Equity in earnings (loss) of joint ventures	(31,852)	52,806	(35,560)	73,241
Net income (loss)	(31,852)	52,031	(35,560)	72,930
Other comprehensive income (loss) before income tax effect	(1,319)	(1,080)	(1,987)	(3,242)
Income tax effect	(9)	(8)	(28)	(25)
Other comprehensive income	(1,328)	(1,088)	(2,015)	(3,267)
Total comprehensive income (loss)	(33,180)	50,943	(37,575)	69,663
The Company [Member] | Reportable Legal Entities [Member]
Revenue, Net [Abstract]
Related parties	0	0	0	0
Other customers	0	0	0	0
Total net sales	0	0	0	0
Cost of goods sold	0	0	0	0
Gross profit (loss)	0	0	0	0
Selling, general and administrative expenses	13,684	9,517	32,798	26,077
Ravenswood (gains) losses	0	0	0	0
Other operating expense - net	0	0	0	0
Operating income (loss)	(13,684)	(9,517)	(32,798)	(26,077)
Interest expense	(5,105)	(5,101)	(15,288)	(15,236)
Intercompany interest	8,909	9,947	27,104	29,222
Interest income	183	33	312	108
Net (loss) on forward and derivative contracts	0	0	0	0
Other income (expense) - net	0	48	677	682
Income (loss) before income taxes and equity in earnings of joint ventures	(9,697)	(4,590)	(19,993)	(11,301)
Income tax (expense) benefit	(3)	(877)	1,097	591
Income (loss) before equity in earnings of joint ventures	(9,700)	(5,467)	(18,896)	(10,710)
Equity in earnings (loss) of joint ventures	30,483	(52,806)	31,671	(73,241)
Net income (loss)	20,783	(58,273)	12,775	(83,951)
Other comprehensive income (loss) before income tax effect	1,429	1,206	3,073	3,618
Income tax effect	(382)	(382)	(1,145)	(1,147)
Other comprehensive income	1,047	824	1,928	2,471
Total comprehensive income (loss)	$ 21,830	(57,449)	$ 14,703	(81,480)
Combined Guarantor Subsidiaries [Member]
Condensed Statement of Income Captions [Line Items]
Ownership percentage	100.00%		100.00%
Combined Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Revenue, Net [Abstract]
Related parties	$ 138,212	170,947	$ 404,130	502,707
Other customers	101,382	32,795	279,061	107,725
Total net sales	239,594	203,742	683,191	610,432
Cost of goods sold	220,723	231,082	655,027	653,258
Gross profit (loss)	18,871	(27,340)	28,164	(42,826)
Selling, general and administrative expenses	0	0	0	0
Ravenswood (gains) losses	0	0	0	0
Other operating expense - net	0	0	0	0
Operating income (loss)	18,871	(27,340)	28,164	(42,826)
Interest expense	(392)	(385)	(1,153)	(1,150)
Intercompany interest	2,214	2,059	6,442	6,013
Interest income	0	0	(1)	9
Net (loss) on forward and derivative contracts	(3,853)	1,275	(17,056)	2,998
Other income (expense) - net	(56)	14	105	29
Income (loss) before income taxes and equity in earnings of joint ventures	16,784	(24,377)	16,501	(34,927)
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in earnings of joint ventures	16,784	(24,377)	16,501	(34,927)
Equity in earnings (loss) of joint ventures	1,369	0	3,889	0
Net income (loss)	18,153	(24,377)	20,390	(34,927)
Other comprehensive income (loss) before income tax effect	982	777	821	2,332
Income tax effect	0	0	0	0
Other comprehensive income	982	777	821	2,332
Total comprehensive income (loss)	19,135	(23,600)	21,211	(32,595)
Combined Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Revenue, Net [Abstract]
Related parties	161,023	152,449	471,949	438,660
Other customers	28	11	93	29
Total net sales	161,051	152,460	472,042	438,689
Cost of goods sold	138,520	141,957	419,493	411,651
Gross profit (loss)	22,531	10,503	52,549	27,038
Selling, general and administrative expenses	310	216	1,250	3,226
Ravenswood (gains) losses	(5,500)	26,830	(5,500)	26,830
Other operating expense - net	437	878	1,590	2,337
Operating income (loss)	27,284	(17,421)	55,209	(5,355)
Interest expense	(45)	(45)	(133)	(135)
Intercompany interest	(11,123)	(12,006)	(33,546)	(35,235)
Interest income	239	157	624	358
Net (loss) on forward and derivative contracts	(35)	0	(12)	0
Other income (expense) - net	483	(219)	(1,847)	(1,173)
Income (loss) before income taxes and equity in earnings of joint ventures	16,803	(29,534)	20,295	(41,540)
Income tax (expense) benefit	(3,318)	1,725	(5,562)	2,646
Income (loss) before equity in earnings of joint ventures	13,485	(27,809)	14,733	(38,894)
Equity in earnings (loss) of joint ventures	214	155	437	891
Net income (loss)	13,699	(27,654)	15,170	(38,003)
Other comprehensive income (loss) before income tax effect	337	303	1,166	910
Income tax effect	9	8	28	25
Other comprehensive income	346	311	1,194	935
Total comprehensive income (loss)	$ 14,045	$ (27,343)	$ 16,364	$ (37,068)