Condensed Guarantor and Non-Guarantor Financial Information, Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017
Current assets:
Cash and cash equivalents	$ 164.0		$ 51.1	$ 57.1
Short-term investments	71.9		183.7
Receivables:
Trade receivables, net	191.2		165.0
Intercompany receivables	0.0		0.0
Other	18.9		15.5
Contract assets	52.6	$ 55.6	0.0
Inventories	177.2	167.2	207.9
Prepaid expenses and other current assets	29.0		33.4
Total current assets	704.8	671.5	656.6
Investments in and advances to subsidiaries	0.0		0.0
Property, plant and equipment, net	584.2		571.4
Long-term intercompany loans receivable	0.0		0.0
Deferred tax assets, net	51.5	68.7	72.0
Intangible assets, net	24.3		25.0
Goodwill	18.8		18.8
Other assets	40.2		41.4
Total	1,423.8	1,396.8	1,385.2
Current liabilities:
Accounts payable	132.0		90.0
Intercompany loans payable	0.0		0.0
Accrued salaries, wages and related expenses	32.2		42.6
Other accrued liabilities	29.9	42.0	40.5
Total current liabilities	194.1	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	62.6		60.0
Long-term debt	370.0		369.6
Total liabilities	662.8	640.4	638.9
Total stockholders' equity	761.0	756.4	746.3
Total	1,423.8	$ 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	(67.2)		(23.6)
Other	0.0		0.0
Contract assets	0.0
Inventories	0.0		0.0
Prepaid expenses and other current assets	(0.4)		0.0
Total current assets	(67.6)		(23.6)
Investments in and advances to subsidiaries	(1,121.4)		(1,145.9)
Property, plant and equipment, net	0.0		0.0
Long-term intercompany loans receivable	(10.5)		(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	(1,194.8)		(1,177.2)
Current liabilities:
Accounts payable	0.0		0.0
Intercompany loans payable	(67.2)		(23.6)
Accrued salaries, wages and related expenses	0.0		0.0
Other accrued liabilities	(3.6)		(9.5)
Total current liabilities	(70.8)		(33.1)
Net liabilities of Salaried VEBA	0.0		0.0
Deferred tax liabilities	0.0		0.0
Long-term intercompany loans payable	(10.5)		(12.4)
Long-term liabilities	0.0		0.0
Long-term debt	0.0		0.0
Total liabilities	(81.3)		(45.5)
Total stockholders' equity	(1,113.5)		(1,131.7)
Total	(1,194.8)		(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	65.6		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	65.7		22.9
Investments in and advances to subsidiaries	1,071.4		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	1,137.1		1,120.6
Current liabilities:
Accounts payable	3.3		1.9
Intercompany loans payable	0.0		0.0
Accrued salaries, wages and related expenses	0.0		0.0
Other accrued liabilities	2.8		2.8
Total current liabilities	6.1		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	370.0		369.6
Total liabilities	376.1		374.3
Total stockholders' equity	761.0		746.3
Total	1,137.1		1,120.6
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	161.0		48.4
Short-term investments	71.9		183.7
Receivables:
Trade receivables, net	183.5		160.1
Intercompany receivables	0.1		0.1
Other	18.5		14.7
Contract assets	49.4
Inventories	169.6		198.7
Prepaid expenses and other current assets	28.4		32.9
Total current assets	682.4		638.6
Investments in and advances to subsidiaries	50.0		48.2
Property, plant and equipment, net	554.6		541.2
Long-term intercompany loans receivable	0.0		0.0
Deferred tax assets, net	46.8		67.3
Intangible assets, net	24.3		25.0
Goodwill	18.8		18.8
Other assets	40.2		41.4
Total	1,417.1		1,380.5
Current liabilities:
Accounts payable	119.8		81.4
Intercompany loans payable	67.1		23.5
Accrued salaries, wages and related expenses	30.7		41.0
Other accrued liabilities	30.6		46.2
Total current liabilities	248.2		192.1
Net liabilities of Salaried VEBA	31.8		31.9
Deferred tax liabilities	0.0		0.0
Long-term intercompany loans payable	10.5		12.4
Long-term liabilities	60.5		58.0
Long-term debt	0.0		0.0
Total liabilities	351.0		294.4
Total stockholders' equity	1,066.1		1,086.1
Total	1,417.1		1,380.5
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	3.0		2.7
Short-term investments	0.0		0.0
Receivables:
Trade receivables, net	7.7		4.9
Intercompany receivables	1.5		0.7
Other	0.4		0.8
Contract assets	3.2
Inventories	7.6		9.2
Prepaid expenses and other current assets	0.9		0.4
Total current assets	24.3		18.7
Investments in and advances to subsidiaries	0.0		0.0
Property, plant and equipment, net	29.6		30.2
Long-term intercompany loans receivable	10.5		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	64.4		61.3
Current liabilities:
Accounts payable	8.9		6.7
Intercompany loans payable	0.1		0.1
Accrued salaries, wages and related expenses	1.5		1.6
Other accrued liabilities	0.1		1.0
Total current liabilities	10.6		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	2.1		2.0
Long-term debt	0.0		0.0
Total liabilities	17.0		15.7
Total stockholders' equity	47.4		45.6
Total	$ 64.4		$ 61.3