Condensed consolidating financial information - Statements of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
NET SALES:
Related parties									$ 1,198,076	$ 1,178,631	$ 1,867,711
Other customers									391,004	140,463	82,146
Total net sales	$ 433,847	$ 400,645	$ 388,802	$ 365,786	$ 339,836	$ 333,650	$ 326,754	$ 318,854	1,589,080	1,319,094	1,949,857
Cost of goods sold									1,460,475	1,330,136	1,908,544
Gross profit (loss)	47,892	41,402	22,460	16,851	5,057	(17,612)	5,582	(3,052)	128,605	(11,042)	41,313
Selling, general and administrative expenses									45,446	40,264	42,115
Helguvik (gains) losses									(7,310)	152,220	0
Ravenswood (gains)									(5,500)	26,830	30,850
Other operating expense - net									2,111	3,857	7,436
Operating income (loss)									93,858	(234,213)	(39,088)
Interest expense - third parties									22,174	22,216	21,954
Interest income (expense) - affiliates									0	0	0
Interest income - third parties									1,397	758	339
Net (loss) gain on forward and derivative contracts				(16,137)					(16,549)	3,487	1,600
Unrealized gain on fair value of contingent consideration									0	0	18,337
Other (expense) income - net									(1,161)	1,319	(356)
Income (loss) before income taxes and equity in earnings of joint ventures									55,371	(250,865)	(41,122)
Income tax (expense) benefit									(7,583)	(2,824)	(9,276)
Income (loss) before equity in earnings of joint ventures									47,788	(253,689)	(50,398)
BHH impairment									0	0	(11,584)
Equity in earnings of joint ventures									792	1,274	2,672
Net income (loss)	$ 35,805	$ 20,783	$ 7,131	$ (15,139)	$ (168,464)	$ (58,273)	$ (9,515)	$ (16,163)	48,580	(252,415)	(59,310)
Other comprehensive income before income tax effect									23,678	288	6,568
Income tax effect									(1,527)	(1,531)	(1,536)
Other comprehensive income									22,151	(1,243)	5,032
Total comprehensive income									$ 70,731	(253,658)	(54,278)
Combined Guarantor Subsidiaries [Member]
Condensed Statement of Income Captions [Line Items]
Ownership percentage	100.00%								100.00%
Reportable Legal Entities [Member] | The Company [Member]
NET SALES:
Related parties									$ 0	0	0
Other customers									0	0	0
Total net sales									0	0	0
Cost of goods sold									0	0	0
Gross profit (loss)									0	0	0
Selling, general and administrative expenses									27,846	20,704	28,000
Helguvik (gains) losses									0	0
Ravenswood (gains)									0	0	0
Other operating expense - net									0	0	0
Operating income (loss)									(27,846)	(20,704)	(28,000)
Interest expense - third parties									20,397	20,378	20,201
Interest income (expense) - affiliates									37,184	39,211	37,626
Interest income - third parties									482	182	65
Net (loss) gain on forward and derivative contracts									0	0	0
Unrealized gain on fair value of contingent consideration											0
Other (expense) income - net									986	1,239	1,356
Income (loss) before income taxes and equity in earnings of joint ventures									(9,591)	(450)	(9,154)
Income tax (expense) benefit									457	1,925	2,140
Income (loss) before equity in earnings of joint ventures									(9,134)	1,475	(7,014)
BHH impairment											0
Equity in earnings of joint ventures									57,714	(253,890)	(52,296)
Net income (loss)									48,580	(252,415)	(59,310)
Other comprehensive income before income tax effect									23,678	288	6,568
Income tax effect									(1,527)	(1,531)	(1,536)
Other comprehensive income									22,151	(1,243)	5,032
Total comprehensive income									70,731	(253,658)	(54,278)
Reportable Legal Entities [Member] | Combined Guarantor Subsidiaries [Member]
NET SALES:
Related parties									547,458	668,486	1,277,474
Other customers									390,875	140,426	82,327
Total net sales									938,333	808,912	1,359,801
Cost of goods sold									888,176	862,632	1,388,400
Gross profit (loss)									50,157	(53,720)	(28,599)
Selling, general and administrative expenses									12,905	9,853	8,798
Helguvik (gains) losses									0	0
Ravenswood (gains)									0	0	0
Other operating expense - net									0	0	0
Operating income (loss)									37,252	(63,573)	(37,397)
Interest expense - third parties									1,554	1,659	1,567
Interest income (expense) - affiliates									8,760	8,107	0
Interest income - third parties									0	10	4
Net (loss) gain on forward and derivative contracts									(17,073)	3,487	1,411
Unrealized gain on fair value of contingent consideration											18,337
Other (expense) income - net									2,976	218	(658)
Income (loss) before income taxes and equity in earnings of joint ventures									30,361	(53,410)	(19,870)
Income tax (expense) benefit									972	0	0
Income (loss) before equity in earnings of joint ventures									31,333	(53,410)	(19,870)
BHH impairment											0
Equity in earnings of joint ventures									2,703	12,463	0
Net income (loss)									34,036	(40,947)	(19,870)
Other comprehensive income before income tax effect									12,722	1,868	14,754
Income tax effect									0	0	0
Other comprehensive income									12,722	1,868	14,754
Total comprehensive income									46,758	(39,079)	(5,116)
Reportable Legal Entities [Member] | Combined Non-Guarantor Subsidiaries [Member]
NET SALES:
Related parties									650,618	510,145	632,597
Other customers									129	37	43
Total net sales									650,747	510,182	632,640
Cost of goods sold									572,299	467,504	564,380
Gross profit (loss)									78,448	42,678	68,260
Selling, general and administrative expenses									4,695	9,707	5,317
Helguvik (gains) losses									(7,310)	152,220
Ravenswood (gains)									(5,500)	26,830	30,850
Other operating expense - net									2,111	3,857	7,436
Operating income (loss)									84,452	(149,936)	24,657
Interest expense - third parties									223	179	186
Interest income (expense) - affiliates									(45,944)	(47,318)	(37,626)
Interest income - third parties									915	566	270
Net (loss) gain on forward and derivative contracts									524	0	189
Unrealized gain on fair value of contingent consideration											0
Other (expense) income - net									(5,123)	(138)	2,356
Income (loss) before income taxes and equity in earnings of joint ventures									34,601	(197,005)	(10,340)
Income tax (expense) benefit									(9,012)	(4,749)	(11,416)
Income (loss) before equity in earnings of joint ventures									25,589	(201,754)	(21,756)
BHH impairment											(11,584)
Equity in earnings of joint ventures									792	1,274	2,672
Net income (loss)									26,381	(200,480)	(30,668)
Other comprehensive income before income tax effect									1,457	5,134	(1,114)
Income tax effect									37	(24)	33
Other comprehensive income									1,494	5,110	(1,081)
Total comprehensive income									27,875	(195,370)	(31,749)
Consolidating Adjustments [Member]
NET SALES:
Related parties									0	0	(42,360)
Other customers									0	0	(224)
Total net sales									0	0	(42,584)
Cost of goods sold									0	0	(44,236)
Gross profit (loss)									0	0	1,652
Selling, general and administrative expenses									0	0	0
Helguvik (gains) losses									0	0
Ravenswood (gains)									0	0	0
Other operating expense - net									0	0	0
Operating income (loss)									0	0	1,652
Interest expense - third parties									0	0	0
Interest income (expense) - affiliates									0	0	0
Interest income - third parties									0	0	0
Net (loss) gain on forward and derivative contracts									0	0	0
Unrealized gain on fair value of contingent consideration											0
Other (expense) income - net									0	0	(3,410)
Income (loss) before income taxes and equity in earnings of joint ventures									0	0	(1,758)
Income tax (expense) benefit									0	0	0
Income (loss) before equity in earnings of joint ventures									0	0	(1,758)
BHH impairment											0
Equity in earnings of joint ventures									(60,417)	241,427	52,296
Net income (loss)									(60,417)	241,427	50,538
Other comprehensive income before income tax effect									(14,179)	(7,002)	(13,640)
Income tax effect									(37)	24	(33)
Other comprehensive income									(14,216)	(6,978)	(13,673)
Total comprehensive income									$ (74,633)	$ 234,449	$ 36,865