Condensed Guarantor and Non-Guarantor Financial Information, Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017
Current assets:
Cash and cash equivalents	$ 94.3		$ 51.1	$ 73.9
Short-term investments	88.4		183.7
Receivables:
Trade receivables, net	194.5		165.0
Intercompany receivables	0.0		0.0
Other	24.4		15.5
Contract assets	51.0	$ 55.6	0.0
Inventories	196.2	167.2	207.9
Prepaid expenses and other current assets	24.2		33.4
Total current assets	673.0	671.5	656.6
Investments in and advances to subsidiaries	0.0		0.0
Property, plant and equipment, net	595.4		571.4
Long-term intercompany loans receivable	0.0		0.0
Deferred tax assets, net	42.2	68.7	72.0
Intangible assets, net	33.0		25.0
Goodwill	43.4		18.8
Other assets	39.0		41.4
Total	1,426.0	1,396.8	1,385.2
Current liabilities:
Accounts payable	120.4		90.0
Intercompany loans payable	0.0		0.0
Accrued salaries, wages and related expenses	35.1		42.6
Other accrued liabilities	37.0	42.0	40.5
Total current liabilities	192.5	174.6	173.1
Net liabilities of Salaried VEBA	31.8		31.9
Deferred tax liabilities	4.3		4.3
Long-term intercompany loans payable	0.0		0.0
Long-term liabilities	64.7		60.0
Long-term debt	370.2		369.6
Total liabilities	663.5	640.4	638.9
Total stockholders' equity	762.5	756.4	746.3
Total	1,426.0	$ 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	(45.3)		(23.6)
Other	0.0		0.0
Contract assets	0.0
Inventories	0.0		0.0
Prepaid expenses and other current assets	0.0		0.0
Total current assets	(45.3)		(23.6)
Investments in and advances to subsidiaries	(1,195.4)		(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	(1,248.4)		(1,177.2)
Current liabilities:
Accounts payable	0.0		0.0
Intercompany loans payable	(45.3)		(23.6)
Accrued salaries, wages and related expenses	0.0		0.0
Other accrued liabilities	(4.8)		(9.5)
Total current liabilities	(50.1)		(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	(62.5)		(45.5)
Total stockholders' equity	(1,185.9)		(1,131.7)
Total	(1,248.4)		(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	43.0		22.8
Other	0.0		0.0
Contract assets	0.0
Inventories	0.0		0.0
Prepaid expenses and other current assets	0.1		0.1
Total current assets	43.1		22.9
Investments in and advances to subsidiaries	1,101.5		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	1,144.6		1,120.6
Current liabilities:
Accounts payable	3.6		1.9
Intercompany loans payable	0.0		0.0
Accrued salaries, wages and related expenses	0.0		0.0
Other accrued liabilities	8.3		2.8
Total current liabilities	11.9		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	370.2		369.6
Total liabilities	382.1		374.3
Total stockholders' equity	762.5		746.3
Total	1,144.6		1,120.6
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	90.4		48.4
Short-term investments	88.4		183.7
Receivables:
Trade receivables, net	188.7		160.1
Intercompany receivables	0.5		0.1
Other	20.1		14.7
Contract assets	48.2
Inventories	183.9		198.7
Prepaid expenses and other current assets	23.7		32.9
Total current assets	643.9		638.6
Investments in and advances to subsidiaries	93.9		48.2
Property, plant and equipment, net	561.4		541.2
Long-term intercompany loans receivable	0.0		0.0
Deferred tax assets, net	37.5		67.3
Intangible assets, net	23.9		25.0
Goodwill	18.8		18.8
Other assets	39.0		41.4
Total	1,418.4		1,380.5
Current liabilities:
Accounts payable	106.3		81.4
Intercompany loans payable	44.9		23.5
Accrued salaries, wages and related expenses	33.6		41.0
Other accrued liabilities	33.1		46.2
Total current liabilities	217.9		192.1
Net liabilities of Salaried VEBA	31.8		31.9
Deferred tax liabilities	0.0		0.0
Long-term intercompany loans payable	12.4		12.4
Long-term liabilities	61.7		58.0
Long-term debt	0.0		0.0
Total liabilities	323.8		294.4
Total stockholders' equity	1,094.6		1,086.1
Total	1,418.4		1,380.5
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	3.9		2.7
Short-term investments	0.0		0.0
Receivables:
Trade receivables, net	5.8		4.9
Intercompany receivables	1.8		0.7
Other	4.3		0.8
Contract assets	2.8
Inventories	12.3		9.2
Prepaid expenses and other current assets	0.4		0.4
Total current assets	31.3		18.7
Investments in and advances to subsidiaries	0.0		0.0
Property, plant and equipment, net	34.0		30.2
Long-term intercompany loans receivable	12.4		12.4
Deferred tax assets, net	0.0		0.0
Intangible assets, net	9.1		0.0
Goodwill	24.6		0.0
Other assets	0.0		0.0
Total	111.4		61.3
Current liabilities:
Accounts payable	10.5		6.7
Intercompany loans payable	0.4		0.1
Accrued salaries, wages and related expenses	1.5		1.6
Other accrued liabilities	0.4		1.0
Total current liabilities	12.8		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	3.0		2.0
Long-term debt	0.0		0.0
Total liabilities	20.1		15.7
Total stockholders' equity	91.3		45.6
Total	$ 111.4		$ 61.3