Condensed Guarantor and Non-Guarantor Financial Information, Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 73.9	$ 55.2	$ 72.9
Short-term investments	191.4	231.0
Receivables:
Trade receivables, net	138.2	137.7
Intercompany receivables	0.0	0.0
Other	15.4	11.9
Inventories	212.2	201.6
Prepaid expenses and other current assets	31.5	18.5
Total current assets	662.6	655.9
Investments in and advances to subsidiaries	0.0	0.0
Property, plant and equipment, net	557.8	530.9
Long-term intercompany loans receivable	0.0	0.0
Deferred tax assets, net	118.7	159.7
Intangible assets, net	25.3	26.4
Goodwill	18.8	37.2		$ 37.2
Other assets	39.5	33.4
Total	1,422.7	1,443.5
Current liabilities:
Accounts payable	94.4	75.8
Intercompany loans payable	0.0	0.0
Accrued salaries, wages and related expenses	39.0	49.1
Other accrued liabilities	43.3	40.1
Total current liabilities	176.7	165.0
Net liabilities of Salaried VEBA	27.8	28.6
Deferred tax liabilities	3.3	3.3
Long-term intercompany loans payable	0.0	0.0
Long-term liabilities	61.9	73.2
Long-term debt	369.4	368.7
Total liabilities	639.1	638.8
Total stockholders' equity	783.6	804.7
Total	1,422.7	1,443.5
Consolidating Adjustments
Current assets:
Cash and cash equivalents	0.0	0.0
Short-term investments	0.0	0.0
Receivables:
Trade receivables, net	0.0	0.0
Intercompany receivables	(56.3)	(86.5)
Other	0.0	0.0
Inventories	(4.9)	(3.9)
Prepaid expenses and other current assets	0.0	(0.5)
Total current assets	(61.2)	(90.9)
Investments in and advances to subsidiaries	(1,148.9)	(1,052.5)
Property, plant and equipment, net	0.0	0.0
Long-term intercompany loans receivable	(10.5)	(85.1)
Deferred tax assets, net	4.8	4.8
Intangible assets, net	0.0	0.0
Goodwill	0.0	0.0
Other assets	0.0	0.0
Total	(1,215.8)	(1,223.7)
Current liabilities:
Accounts payable	0.0	0.0
Intercompany loans payable	(56.3)	(86.5)
Accrued salaries, wages and related expenses	0.0	0.0
Other accrued liabilities	(7.1)	(14.7)
Total current liabilities	(63.4)	(101.2)
Net liabilities of Salaried VEBA	0.0	0.0
Deferred tax liabilities	0.0	0.0
Long-term intercompany loans payable	(10.5)	(85.1)
Long-term liabilities	0.0	0.0
Long-term debt	0.0	0.0
Total liabilities	(73.9)	(186.3)
Total stockholders' equity	(1,141.9)	(1,037.4)
Total	(1,215.8)	(1,223.7)
Parent
Current assets:
Cash and cash equivalents	0.0	0.0
Short-term investments	0.0	0.0
Receivables:
Trade receivables, net	0.0	0.0
Intercompany receivables	55.6	85.8
Other	0.0	0.0
Inventories	0.0	0.0
Prepaid expenses and other current assets	0.1	0.1
Total current assets	55.7	85.9
Investments in and advances to subsidiaries	1,107.0	1,012.4
Property, plant and equipment, net	0.0	0.0
Long-term intercompany loans receivable	0.0	80.2
Deferred tax assets, net	0.0	0.0
Intangible assets, net	0.0	0.0
Goodwill	0.0	0.0
Other assets	0.0	0.0
Total	1,162.7	1,178.5
Current liabilities:
Accounts payable	1.4	2.2
Intercompany loans payable	0.0	0.0
Accrued salaries, wages and related expenses	0.0	0.0
Other accrued liabilities	8.3	2.9
Total current liabilities	9.7	5.1
Net liabilities of Salaried VEBA	0.0	0.0
Deferred tax liabilities	0.0	0.0
Long-term intercompany loans payable	0.0	0.0
Long-term liabilities	0.0	0.0
Long-term debt	369.4	368.7
Total liabilities	379.1	373.8
Total stockholders' equity	783.6	804.7
Total	1,162.7	1,178.5
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	70.7	52.9
Short-term investments	191.4	231.0
Receivables:
Trade receivables, net	133.1	133.1
Intercompany receivables	0.1	0.1
Other	14.6	11.4
Inventories	208.5	197.5
Prepaid expenses and other current assets	31.0	18.0
Total current assets	649.4	644.0
Investments in and advances to subsidiaries	41.9	40.1
Property, plant and equipment, net	527.6	499.5
Long-term intercompany loans receivable	0.0	0.0
Deferred tax assets, net	113.9	154.9
Intangible assets, net	25.3	26.4
Goodwill	18.8	37.2
Other assets	39.5	33.4
Total	1,416.4	1,435.5
Current liabilities:
Accounts payable	86.8	68.9
Intercompany loans payable	56.2	86.4
Accrued salaries, wages and related expenses	37.2	47.2
Other accrued liabilities	41.0	52.6
Total current liabilities	221.2	255.1
Net liabilities of Salaried VEBA	27.8	28.6
Deferred tax liabilities	0.0	0.0
Long-term intercompany loans payable	10.5	85.1
Long-term liabilities	59.3	70.5
Long-term debt	0.0	0.0
Total liabilities	318.8	439.3
Total stockholders' equity	1,097.6	996.2
Total	1,416.4	1,435.5
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	3.2	2.3
Short-term investments	0.0	0.0
Receivables:
Trade receivables, net	5.1	4.6
Intercompany receivables	0.6	0.6
Other	0.8	0.5
Inventories	8.6	8.0
Prepaid expenses and other current assets	0.4	0.9
Total current assets	18.7	16.9
Investments in and advances to subsidiaries	0.0	0.0
Property, plant and equipment, net	30.2	31.4
Long-term intercompany loans receivable	10.5	4.9
Deferred tax assets, net	0.0	0.0
Intangible assets, net	0.0	0.0
Goodwill	0.0	0.0
Other assets	0.0	0.0
Total	59.4	53.2
Current liabilities:
Accounts payable	6.2	4.7
Intercompany loans payable	0.1	0.1
Accrued salaries, wages and related expenses	1.8	1.9
Other accrued liabilities	1.1	(0.7)
Total current liabilities	9.2	6.0
Net liabilities of Salaried VEBA	0.0	0.0
Deferred tax liabilities	3.3	3.3
Long-term intercompany loans payable	0.0	0.0
Long-term liabilities	2.6	2.7
Long-term debt	0.0	0.0
Total liabilities	15.1	12.0
Total stockholders' equity	44.3	41.2
Total	$ 59.4	$ 53.2