Condensed Guarantor and Non-Guarantor Financial Information, Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017
Current assets:
Cash and cash equivalents	$ 135.0		$ 51.1	$ 34.4
Short-term investments	83.8		183.7
Receivables:
Trade receivables, net	185.3		165.0
Intercompany receivables	0.0		0.0
Other	18.2		15.5
Contract with Customer, Asset, Net, Current	57.5	$ 55.6	0.0
Inventories	176.8	167.2	207.9
Prepaid expenses and other current assets	25.7		33.4
Total current assets	682.3	671.5	656.6
Investments in and advances to subsidiaries	0.0		0.0
Property, plant and equipment, net	578.7		571.4
Long-term intercompany loans receivable	0.0		0.0
Deferred tax assets, net	63.7	68.7	72.0
Intangible assets, net	24.6		25.0
Goodwill	18.8		18.8
Other assets	37.3		41.4
Total	1,405.4	1,396.8	1,385.2
Current liabilities:
Accounts payable	120.2		90.0
Intercompany loans payable	0.0		0.0
Accrued salaries, wages and related expenses	28.7		42.6
Other accrued liabilities	35.6	42.0	40.5
Total current liabilities	184.5	174.6	173.1
Net liabilities of Salaried VEBA	31.9		31.9
Deferred tax liabilities	4.3		4.3
Long-term intercompany loans payable	0.0		0.0
Long-term liabilities	61.3		60.0
Long-term debt	369.8		369.6
Total liabilities	651.8	640.4	638.9
Total stockholders' equity	753.6	756.4	746.3
Total	1,405.4	$ 1,396.8	1,385.2
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	(100.6)		(23.6)
Other	0.0		0.0
Contract with Customer, Asset, Net, Current	0.0
Inventories	0.0		0.0
Prepaid expenses and other current assets	0.0		0.0
Total current assets	(100.6)		(23.6)
Investments in and advances to subsidiaries	(1,083.5)		(1,145.9)
Property, plant and equipment, net	0.0		0.0
Long-term intercompany loans receivable	(12.4)		(12.4)
Deferred tax assets, net	4.7		4.7
Intangible assets, net	0.0		0.0
Goodwill	0.0		0.0
Other assets	0.0		0.0
Total	(1,191.8)		(1,177.2)
Current liabilities:
Accounts payable	0.0		0.0
Intercompany loans payable	(100.6)		(23.6)
Accrued salaries, wages and related expenses	0.0		0.0
Other accrued liabilities	(1.6)		(9.5)
Total current liabilities	(102.2)		(33.1)
Net liabilities of Salaried VEBA	0.0		0.0
Deferred tax liabilities	0.0		0.0
Long-term intercompany loans payable	(12.4)		(12.4)
Long-term liabilities	0.0		0.0
Long-term debt	0.0		0.0
Total liabilities	(114.6)		(45.5)
Total stockholders' equity	(1,077.2)		(1,131.7)
Total	(1,191.8)		(1,177.2)
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	99.9		22.8
Other	0.0		0.0
Contract with Customer, Asset, Net, Current	0.0
Inventories	0.0		0.0
Prepaid expenses and other current assets	0.1		0.1
Total current assets	100.0		22.9
Investments in and advances to subsidiaries	1,034.5		1,097.7
Property, plant and equipment, net	0.0		0.0
Long-term intercompany loans receivable	0.0		0.0
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	1,134.5		1,120.6
Current liabilities:
Accounts payable	2.7		1.9
Intercompany loans payable	0.0		0.0
Accrued salaries, wages and related expenses	0.0		0.0
Other accrued liabilities	8.4		2.8
Total current liabilities	11.1		4.7
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	369.8		369.6
Total liabilities	380.9		374.3
Total stockholders' equity	753.6		746.3
Total	1,134.5		1,120.6
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	131.7		48.4
Short-term investments	83.8		183.7
Receivables:
Trade receivables, net	178.5		160.1
Intercompany receivables	0.1		0.1
Other	17.6		14.7
Contract with Customer, Asset, Net, Current	57.5
Inventories	166.5		198.7
Prepaid expenses and other current assets	25.2		32.9
Total current assets	660.9		638.6
Investments in and advances to subsidiaries	49.0		48.2
Property, plant and equipment, net	548.8		541.2
Long-term intercompany loans receivable	0.0		0.0
Deferred tax assets, net	59.0		67.3
Intangible assets, net	24.6		25.0
Goodwill	18.8		18.8
Other assets	37.3		41.4
Total	1,398.4		1,380.5
Current liabilities:
Accounts payable	109.0		81.4
Intercompany loans payable	100.5		23.5
Accrued salaries, wages and related expenses	27.2		41.0
Other accrued liabilities	27.4		46.2
Total current liabilities	264.1		192.1
Net liabilities of Salaried VEBA	31.9		31.9
Deferred tax liabilities	0.0		0.0
Long-term intercompany loans payable	12.4		12.4
Long-term liabilities	59.1		58.0
Long-term debt	0.0		0.0
Total liabilities	367.5		294.4
Total stockholders' equity	1,030.9		1,086.1
Total	1,398.4		1,380.5
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	3.3		2.7
Short-term investments	0.0		0.0
Receivables:
Trade receivables, net	6.8		4.9
Intercompany receivables	0.6		0.7
Other	0.6		0.8
Contract with Customer, Asset, Net, Current	0.0
Inventories	10.3		9.2
Prepaid expenses and other current assets	0.4		0.4
Total current assets	22.0		18.7
Investments in and advances to subsidiaries	0.0		0.0
Property, plant and equipment, net	29.9		30.2
Long-term intercompany loans receivable	12.4		12.4
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	64.3		61.3
Current liabilities:
Accounts payable	8.5		6.7
Intercompany loans payable	0.1		0.1
Accrued salaries, wages and related expenses	1.5		1.6
Other accrued liabilities	1.4		1.0
Total current liabilities	11.5		9.4
Net liabilities of Salaried VEBA	0.0		0.0
Deferred tax liabilities	4.3		4.3
Long-term intercompany loans payable	0.0		0.0
Long-term liabilities	2.2		2.0
Long-term debt	0.0		0.0
Total liabilities	18.0		15.7
Total stockholders' equity	46.3		45.6
Total	$ 64.3		$ 61.3