Condensed consolidating financial information - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	$ 174,213	$ 132,403	$ 117,593	$ 115,393
Restricted cash	848	1,050
Accounts receivable - net	40,367	12,432
Due from affiliates	11,849	16,651
Inventories	261,756	233,563
Prepaid and other current assets	21,761	22,210
Assets held for sale	0	22,313
Total current assets	510,794	440,622
Property, plant and equipment - net	984,948	1,026,285
Investment in subsidiaries	0	0
Due from affiliates - long term	0	0
Other assets	67,721	73,420
TOTAL	1,563,463	1,540,327
Accounts payable, trade	97,032	94,960
Due to affiliates	23,657	15,368
Accrued and other current liabilities	56,732	50,100
Accrued employee benefits costs	10,444	10,917
Industrial revenue bonds	7,815	7,815
Total current liabilities	195,680	179,160
Senior notes payable	248,036	247,699
Accrued pension benefits costs - less current portion	46,835	49,493
Accrued postretirement benefits costs - less current portion	128,092	126,355
Due to affiliates - long term	0
Other liabilities	62,094	72,026
Intercompany loan		0
Deferred taxes	109,632	108,939
Total noncurrent liabilities	594,689	604,512
Preferred stock	1	1
Common stock	945	944
Other shareholders' equity	772,148	755,710
Total shareholders’ equity	773,094	756,655
TOTAL	1,563,463	1,540,327
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	0	0
Prepaid and other current assets	0	0
Assets held for sale		0
Total current assets	0	0
Property, plant and equipment - net	0	0
Investment in subsidiaries	(791,474)	(753,899)
Due from affiliates - long term	(903,437)	(878,558)
Other assets	(31,915)	(31,722)
TOTAL	(1,726,826)	(1,664,179)
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 - less current portion	(31,915)	(31,722)
Accrued postretirement benefits costs - less current portion	0	0
Due to affiliates - long term	(903,437)
Other liabilities	0	0
Intercompany loan		(878,558)
Deferred taxes	0	0
Total noncurrent liabilities	(935,352)	(910,280)
Preferred stock	0	0
Common stock	(60)	(60)
Other shareholders' equity	(791,414)	(753,839)
Total shareholders’ equity	(791,474)	(753,899)
TOTAL	(1,726,826)	(1,664,179)
The Company [Member] | Reportable Legal Entities [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	70,752	36,670	37,605	58,421
Restricted cash	0	0
Accounts receivable - net	0	167
Due from affiliates	42	42
Inventories	180	180
Prepaid and other current assets	5,959	6,838
Assets held for sale		0
Total current assets	76,933	43,897
Property, plant and equipment - net	17,970	12,311
Investment in subsidiaries	736,345	702,659
Due from affiliates - long term	514,676	529,873
Other assets	27,834	28,215
TOTAL	1,373,758	1,316,955
Accounts payable, trade	2,712	6,125
Due to affiliates	785	417
Accrued and other current liabilities	18,965	11,950
Accrued employee benefits costs	1,933	1,932
Industrial revenue bonds	0	0
Total current liabilities	24,395	20,424
Senior notes payable	248,036	247,699
Accrued pension benefits costs - less current portion	45,587	46,390
Accrued postretirement benefits costs - less current portion	3,873	4,380
Due to affiliates - long term	270,878
Other liabilities	7,895	4,160
Intercompany loan		237,247
Deferred taxes	0	0
Total noncurrent liabilities	576,269	539,876
Preferred stock	1	1
Common stock	945	944
Other shareholders' equity	772,148	755,710
Total shareholders’ equity	773,094	756,655
TOTAL	1,373,758	1,316,955
Combined Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	(217)	(255)	(125)	(3,647)
Restricted cash	800	794
Accounts receivable - net	40,117	11,883
Due from affiliates	11,806	16,606
Inventories	165,288	146,689
Prepaid and other current assets	6,176	5,699
Assets held for sale		0
Total current assets	223,970	181,416
Property, plant and equipment - net	303,708	328,069
Investment in subsidiaries	55,129	51,240
Due from affiliates - long term	379,331	346,893
Other assets	43,915	49,331
TOTAL	1,006,053	956,949
Accounts payable, trade	53,785	52,921
Due to affiliates	15,902	9,641
Accrued and other current liabilities	19,035	17,744
Accrued employee benefits costs	8,020	8,317
Industrial revenue bonds	7,815	7,815
Total current liabilities	104,557	96,438
Senior notes payable	0	0
Accrued pension benefits costs - less current portion	19,666	20,167
Accrued postretirement benefits costs - less current portion	122,549	120,242
Due to affiliates - long term	59,395
Other liabilities	32,115	30,920
Intercompany loan		42,609
Deferred taxes	2,721	2,735
Total noncurrent liabilities	236,446	216,673
Preferred stock	0	0
Common stock	1	1
Other shareholders' equity	665,049	643,837
Total shareholders’ equity	665,050	643,838
TOTAL	1,006,053	956,949
Combined Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	103,678	95,988	$ 80,113	$ 60,619
Restricted cash	48	256
Accounts receivable - net	250	382
Due from affiliates	1	3
Inventories	96,288	86,694
Prepaid and other current assets	9,626	9,673
Assets held for sale		22,313
Total current assets	209,891	215,309
Property, plant and equipment - net	663,270	685,905
Investment in subsidiaries	0	0
Due from affiliates - long term	9,430	1,792
Other assets	27,887	27,596
TOTAL	910,478	930,602
Accounts payable, trade	40,535	35,914
Due to affiliates	6,970	5,310
Accrued and other current liabilities	18,732	20,406
Accrued employee benefits costs	491	668
Industrial revenue bonds	0	0
Total current liabilities	66,728	62,298
Senior notes payable	0	0
Accrued pension benefits costs - less current portion	13,497	14,658
Accrued postretirement benefits costs - less current portion	1,670	1,733
Due to affiliates - long term	573,164
Other liabilities	22,084	36,946
Intercompany loan		598,702
Deferred taxes	106,911	106,204
Total noncurrent liabilities	717,326	758,243
Preferred stock	0	0
Common stock	59	59
Other shareholders' equity	126,365	110,002
Total shareholders’ equity	126,424	110,061
TOTAL	$ 910,478	$ 930,602