Condensed consolidating financial information - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	$ 129,384	$ 115,393	$ 167,408	$ 163,242
Restricted cash	2,669	791
Accounts receivable - net	14,304	9,475
Due from affiliates	14,511	17,417
Inventories	231,832	231,872
Prepaid and other current assets	27,953	42,412
Assets held for sale	29,695	30,697
Total current assets	450,348	448,057
Property, plant and equipment - net	1,197,604	1,232,256
Investment in subsidiaries	0	0
Due from affiliates - less current portion	0	0
Other assets	71,636	72,155
TOTAL	1,719,588	1,752,468
Accounts payable, trade	89,303	90,489
Due to affiliates	9,662	10,045
Accrued and other current liabilities	48,479	48,822
Accrued employee benefits costs	10,006	10,148
Industrial revenue bonds	7,815	7,815
Total current liabilities	165,265	167,319
Senior notes payable	247,484	247,278
Accrued pension benefits costs cost - less current portion	42,935	43,999
Accrued postretirement benefits costs - less current portion	125,903	125,999
Other liabilities	53,102	53,009
Intercompany loan	0	0
Deferred taxes	90,464	96,994
Total noncurrent liabilities	559,888	567,279
Series A Preferred stock	1	1
Common stock	942	942
Additional paid-in capital	2,514,227	2,513,631
Treasury stock, at cost	(86,276)	(86,276)
Accumulated other comprehensive loss	(111,003)	(112,650)
Accumulated deficit	(1,323,456)	(1,297,778)
Total shareholders’ equity	994,435	1,017,870
TOTAL	1,719,588	1,752,468
Consolidating Adjustments [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	0	0	0	0
Restricted cash	0	0
Accounts receivable - net	0	0
Due from affiliates	0	0
Inventories	(3,079)	(2,525)
Prepaid and other current assets	0	0
Assets held for sale	0	0
Total current assets	(3,079)	(2,525)
Property, plant and equipment - net	0	0
Investment in subsidiaries	(574,884)	(593,604)
Due from affiliates - less current portion	(639,889)	(632,170)
Other assets	7,418	7,419
TOTAL	(1,210,434)	(1,220,880)
Accounts payable, trade	0	0
Due to affiliates	0	0
Accrued and other current liabilities	0	0
Accrued employee benefits costs	0	0
Industrial revenue bonds	0	0
Total current liabilities	0	0
Senior notes payable	0	0
Accrued pension benefits costs cost - less current portion	0	0
Accrued postretirement benefits costs - less current portion	0	0
Other liabilities	7,418	7,418
Intercompany loan	(643,039)	(634,170)
Deferred taxes	0	0
Total noncurrent liabilities	(635,621)	(626,752)
Series A Preferred stock	0	0
Common stock	(228,424)	(224,424)
Additional paid-in capital	(2,373,556)	(2,229,161)
Treasury stock, at cost	0	0
Accumulated other comprehensive loss	0	0
Accumulated deficit	2,027,167	1,859,457
Total shareholders’ equity	(574,813)	(594,128)
TOTAL	(1,210,434)	(1,220,880)
The Company [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	47,316	58,421	125,424	70,683
Restricted cash	0	0
Accounts receivable - net	374	104
Due from affiliates	138	13
Inventories	180	180
Prepaid and other current assets	6,383	7,713
Assets held for sale	0	0
Total current assets	54,391	66,431
Property, plant and equipment - net	8,532	9,188
Investment in subsidiaries	574,884	593,604
Due from affiliates - less current portion	635,612	632,170
Other assets	29,094	29,536
TOTAL	1,302,513	1,330,929
Accounts payable, trade	1,099	2,380
Due to affiliates	200	2,143
Accrued and other current liabilities	10,731	11,247
Accrued employee benefits costs	1,824	1,824
Industrial revenue bonds	0	0
Total current liabilities	13,854	17,594
Senior notes payable	247,484	247,278
Accrued pension benefits costs cost - less current portion	39,300	39,831
Accrued postretirement benefits costs - less current portion	5,030	4,524
Other liabilities	2,482	3,307
Intercompany loan	0	0
Deferred taxes	0	0
Total noncurrent liabilities	294,296	294,940
Series A Preferred stock	1	1
Common stock	942	942
Additional paid-in capital	2,514,227	2,513,631
Treasury stock, at cost	(86,276)	(86,276)
Accumulated other comprehensive loss	(13,657)	(13,125)
Accumulated deficit	(1,420,874)	(1,396,778)
Total shareholders’ equity	994,363	1,018,395
TOTAL	1,302,513	1,330,929
Combined Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	(278)	(3,647)	(661)	(1,187)
Restricted cash	795	791
Accounts receivable - net	13,473	8,891
Due from affiliates	12,604	14,493
Inventories	146,841	148,280
Prepaid and other current assets	4,699	16,242
Assets held for sale	0	0
Total current assets	178,134	185,050
Property, plant and equipment - net	343,026	361,626
Investment in subsidiaries	0	0
Due from affiliates - less current portion	4,277	0
Other assets	18,288	19,153
TOTAL	543,725	565,829
Accounts payable, trade	51,612	53,145
Due to affiliates	4,478	7,167
Accrued and other current liabilities	14,107	14,759
Accrued employee benefits costs	7,502	7,644
Industrial revenue bonds	7,815	7,815
Total current liabilities	85,514	90,530
Senior notes payable	0	0
Accrued pension benefits costs cost - less current portion	(10,964)	(11,021)
Accrued postretirement benefits costs - less current portion	119,282	119,911
Other liabilities	31,484	30,505
Intercompany loan	27,995	124,518
Deferred taxes	0	0
Total noncurrent liabilities	167,797	263,913
Series A Preferred stock	0	0
Common stock	1	0
Additional paid-in capital	297,899	191,023
Treasury stock, at cost	0	0
Accumulated other comprehensive loss	(58,471)	(60,026)
Accumulated deficit	50,985	80,389
Total shareholders’ equity	290,414	211,386
TOTAL	543,725	565,829
Combined Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	82,346	60,619	$ 42,645	$ 93,746
Restricted cash	1,874	0
Accounts receivable - net	457	480
Due from affiliates	1,769	2,911
Inventories	87,890	85,937
Prepaid and other current assets	16,871	18,457
Assets held for sale	29,695	30,697
Total current assets	220,902	199,101
Property, plant and equipment - net	846,046	861,442
Investment in subsidiaries	0	0
Due from affiliates - less current portion	0	0
Other assets	16,836	16,047
TOTAL	1,083,784	1,076,590
Accounts payable, trade	36,592	34,964
Due to affiliates	4,984	735
Accrued and other current liabilities	23,641	22,816
Accrued employee benefits costs	680	680
Industrial revenue bonds	0	0
Total current liabilities	65,897	59,195
Senior notes payable	0	0
Accrued pension benefits costs cost - less current portion	14,599	15,189
Accrued postretirement benefits costs - less current portion	1,591	1,564
Other liabilities	11,718	11,779
Intercompany loan	615,044	509,652
Deferred taxes	90,464	96,994
Total noncurrent liabilities	733,416	635,178
Series A Preferred stock	0	0
Common stock	228,423	224,424
Additional paid-in capital	2,075,657	2,038,138
Treasury stock, at cost	0	0
Accumulated other comprehensive loss	(38,875)	(39,499)
Accumulated deficit	(1,980,734)	(1,840,846)
Total shareholders’ equity	284,471	382,217
TOTAL	$ 1,083,784	$ 1,076,590