Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Cash and cash equivalents	$ 38.9	$ 167.2
Restricted cash	0.8	0.8
Accounts receivable - net	82.5	43.1
Due from affiliates	22.7	10.4
Inventories	343.8	317.5
Prepaid and other current assets	18.0	14.7
Total current assets	506.7	553.7
Property, plant and equipment - net	967.3	971.9
Investment in subsidiaries	0.0	0.0
Due from affiliates - less current portion	0.0	0.0
Other assets	63.5	56.0
TOTAL	1,537.5	1,581.6	$ 1,540.3
LIABILITIES:
Accounts payable, trade	119.4	89.9
Due to affiliates	10.3	20.4
Accrued and other current liabilities	52.5	61.4
Accrued employee benefits costs	11.0	11.0
Revolving credit facility	23.3	0.0
Industrial revenue bonds	7.8	7.8
Total current liabilities	224.3	190.5
Senior notes payable	248.6	248.2
Accrued pension benefits costs - less current portion	50.9	38.9
Accrued postretirement benefits costs - less current portion	101.2	113.0
Other liabilities	46.0	57.9
Intercompany loan	0.0	0.0
Deferred taxes	104.3	103.5
Total noncurrent liabilities	551.0	561.5
SHAREHOLDERS’ EQUITY:
Preferred stock	0.0	0.0
Common stock	1.0	0.9
Other shareholders' equity	761.2	828.7
Total shareholders’ equity	762.2	829.6	$ 756.7	$ 1,008.8
TOTAL	1,537.5	1,581.6
Reportable Legal Entities [Member] | The Company [Member]
ASSETS
Cash and cash equivalents	0.1	64.3
Restricted cash	0.0	0.0
Accounts receivable - net	0.5	0.0
Due from affiliates	0.0	0.1
Inventories	0.0	0.2
Prepaid and other current assets	6.4	3.3
Total current assets	7.0	67.9
Property, plant and equipment - net	20.6	19.4
Investment in subsidiaries	668.3	751.8
Due from affiliates - less current portion	751.7	513.3
Other assets	29.8	28.0
TOTAL	1,477.4	1,380.4
LIABILITIES:
Accounts payable, trade	3.7	6.3
Due to affiliates	0.0	0.4
Accrued and other current liabilities	15.8	16.0
Accrued employee benefits costs	1.9	1.9
Revolving credit facility	23.3
Industrial revenue bonds	0.0	0.0
Total current liabilities	44.7	24.6
Senior notes payable	248.6	248.2
Accrued pension benefits costs - less current portion	23.2	39.5
Accrued postretirement benefits costs - less current portion	0.7	0.9
Other liabilities	2.8	3.5
Intercompany loan	395.4	234.1
Deferred taxes	(0.2)	0.0
Total noncurrent liabilities	670.5	526.2
SHAREHOLDERS’ EQUITY:
Preferred stock	0.0	0.0
Common stock	1.0	0.9
Other shareholders' equity	761.2	828.7
Total shareholders’ equity	762.2	829.6
TOTAL	1,477.4	1,380.4
Reportable Legal Entities [Member] | Combined Guarantor Subsidiaries [Member]
ASSETS
Cash and cash equivalents	0.0	(0.1)
Restricted cash	0.8	0.8
Accounts receivable - net	81.8	42.8
Due from affiliates	13.1	10.3
Inventories	210.7	205.1
Prepaid and other current assets	3.4	0.8
Total current assets	309.8	259.7
Property, plant and equipment - net	320.7	295.8
Investment in subsidiaries	54.5	54.0
Due from affiliates - less current portion	517.6	349.6
Other assets	2.1	34.1
TOTAL	1,204.7	993.2
LIABILITIES:
Accounts payable, trade	84.1	51.4
Due to affiliates	0.0	2.6
Accrued and other current liabilities	22.8	19.3
Accrued employee benefits costs	8.3	8.3
Revolving credit facility	0.0
Industrial revenue bonds	7.8	7.8
Total current liabilities	123.0	89.4
Senior notes payable	0.0	0.0
Accrued pension benefits costs - less current portion	20.7	18.3
Accrued postretirement benefits costs - less current portion	98.9	110.4
Other liabilities	23.5	32.0
Intercompany loan	307.6	53.8
Deferred taxes	1.8	1.7
Total noncurrent liabilities	452.5	216.2
SHAREHOLDERS’ EQUITY:
Preferred stock	0.0	0.0
Common stock	0.0	0.0
Other shareholders' equity	629.2	687.6
Total shareholders’ equity	629.2	687.6
TOTAL	1,204.7	993.2
Reportable Legal Entities [Member] | Combined Non-Guarantor Subsidiaries [Member]
ASSETS
Cash and cash equivalents	38.8	103.0
Restricted cash	0.0	0.0
Accounts receivable - net	0.2	0.3
Due from affiliates	9.6	0.0
Inventories	133.1	112.2
Prepaid and other current assets	8.2	10.6
Total current assets	189.9	226.1
Property, plant and equipment - net	626.0	656.7
Investment in subsidiaries	0.0	0.0
Due from affiliates - less current portion	7.2	9.4
Other assets	31.6	25.9
TOTAL	854.7	918.1
LIABILITIES:
Accounts payable, trade	31.6	32.2
Due to affiliates	10.3	17.4
Accrued and other current liabilities	13.9	26.1
Accrued employee benefits costs	0.8	0.8
Revolving credit facility	0.0
Industrial revenue bonds	0.0	0.0
Total current liabilities	56.6	76.5
Senior notes payable	0.0	0.0
Accrued pension benefits costs - less current portion	7.0	13.1
Accrued postretirement benefits costs - less current portion	1.6	1.7
Other liabilities	19.7	22.4
Intercompany loan	573.5	584.4
Deferred taxes	102.7	101.8
Total noncurrent liabilities	704.5	723.4
SHAREHOLDERS’ EQUITY:
Preferred stock	0.0	0.0
Common stock	0.1	0.1
Other shareholders' equity	93.5	118.1
Total shareholders’ equity	93.6	118.2
TOTAL	854.7	918.1
Consolidating Adjustments [Member]
ASSETS
Cash and cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Accounts receivable - net	0.0	0.0
Due from affiliates	0.0	0.0
Inventories	0.0	0.0
Prepaid and other current assets	0.0	0.0
Total current assets	0.0	0.0
Property, plant and equipment - net	0.0	0.0
Investment in subsidiaries	(722.8)	(805.8)
Due from affiliates - less current portion	(1,276.5)	(872.3)
Other assets	0.0	(32.0)
TOTAL	(1,999.3)	(1,710.1)
LIABILITIES:
Accounts payable, trade	0.0	0.0
Due to affiliates	0.0	0.0
Accrued and other current liabilities	0.0	0.0
Accrued employee benefits costs	0.0	0.0
Revolving credit facility	0.0
Industrial revenue bonds	0.0	0.0
Total current liabilities	0.0	0.0
Senior notes payable	0.0	0.0
Accrued pension benefits costs - less current portion	0.0	(32.0)
Accrued postretirement benefits costs - less current portion	0.0	0.0
Other liabilities	0.0	0.0
Intercompany loan	(1,276.5)	(872.3)
Deferred taxes	0.0	0.0
Total noncurrent liabilities	(1,276.5)	(904.3)
SHAREHOLDERS’ EQUITY:
Preferred stock	0.0	0.0
Common stock	(0.1)	(0.1)
Other shareholders' equity	(722.7)	(805.7)
Total shareholders’ equity	(722.8)	(805.8)
TOTAL	$ (1,999.3)	$ (1,710.1)