Condensed Guarantor and Non-Guarantor Financial Information, Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 122.4		$ 125.6	$ 164.0
Short-term investments	24.4		36.7
Receivables:
Trade receivables, net	178.1		179.8
Intercompany receivables	0.0		0.0
Other	25.1		25.6
Contract assets	46.9		54.9
Inventories	231.1		215.1
Prepaid expenses and other current assets	21.7		18.9
Total current assets	649.7		656.6
Investments in and advances to subsidiaries	0.0		0.0
Property, plant and equipment, net	613.8		611.8
Operating lease assets	26.8		0.0
Long-term intercompany loans receivable	0.0		0.0
Deferred tax assets, net	15.5		35.9
Intangible assets, net	31.0		32.4
Goodwill	44.0		44.0
Other assets	41.3		38.6
Total	1,422.1		1,419.3
Current liabilities:
Accounts payable	101.3		121.4
Intercompany loans payable	0.0		0.0
Accrued salaries, wages and related expenses	30.4		40.1
Other accrued liabilities	47.2		44.0
Total current liabilities	178.9		205.5
Long-term portion of operating lease liabilities	26.0		0.0
Net liabilities of Salaried VEBA	32.6		32.4
Deferred tax liabilities	4.2		4.2
Long-term intercompany loans payable	0.0		0.0
Long-term liabilities	63.2		66.4
Long-term debt	370.9		370.4
Total liabilities	675.8		678.9
Total stockholders' equity	746.3	$ 747.9	740.4	$ 761.0	$ 753.6	$ 746.3
Total	1,422.1		1,419.3
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	(79.0)		(148.1)
Other	0.0		0.0
Contract assets	0.0		0.0
Inventories	0.0		0.0
Prepaid expenses and other current assets	0.0		0.0
Total current assets	(79.0)		(148.1)
Investments in and advances to subsidiaries	(1,143.1)		(1,069.8)
Property, plant and equipment, net	0.0		0.0
Operating lease assets	0.0
Long-term intercompany loans receivable	(10.4)		(9.8)
Deferred tax assets, net	3.1		3.0
Intangible assets, net	0.0		0.0
Goodwill	0.0		0.0
Other assets	0.0		0.0
Total	(1,229.4)		(1,224.7)
Current liabilities:
Accounts payable	0.0		0.0
Intercompany loans payable	(79.0)		(148.1)
Accrued salaries, wages and related expenses	0.0		0.0
Other accrued liabilities	(3.1)		(6.3)
Total current liabilities	(82.1)		(154.4)
Long-term portion of operating lease liabilities	0.0
Net liabilities of Salaried VEBA	0.0		0.0
Deferred tax liabilities	0.0		0.0
Long-term intercompany loans payable	(10.4)		(9.8)
Long-term liabilities	0.0		0.0
Long-term debt	0.0		0.0
Total liabilities	(92.5)		(164.2)
Total stockholders' equity	(1,136.9)		(1,060.5)
Total	(1,229.4)		(1,224.7)
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	74.0		143.7
Other	0.0		0.0
Contract assets	0.0		0.0
Inventories	0.0		0.0
Prepaid expenses and other current assets	0.0		0.1
Total current assets	74.0		143.8
Investments in and advances to subsidiaries	1,047.7		974.7
Property, plant and equipment, net	0.0		0.0
Operating lease assets	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	1,121.7		1,118.5
Current liabilities:
Accounts payable	1.7		4.9
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	4.5		7.7
Long-term portion of operating lease liabilities	0.0
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	370.9		370.4
Total liabilities	375.4		378.1
Total stockholders' equity	746.3		740.4
Total	1,121.7		1,118.5
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	120.8		122.6
Short-term investments	24.4		36.7
Receivables:
Trade receivables, net	173.0		174.0
Intercompany receivables	0.5		0.1
Other	24.2		23.3
Contract assets	44.8		52.0
Inventories	216.3		203.0
Prepaid expenses and other current assets	21.1		18.4
Total current assets	625.1		630.1
Investments in and advances to subsidiaries	95.2		94.9
Property, plant and equipment, net	579.8		577.4
Operating lease assets	26.8
Long-term intercompany loans receivable	0.0		0.0
Deferred tax assets, net	12.4		32.9
Intangible assets, net	22.9		23.6
Goodwill	18.8		18.8
Other assets	41.3		38.6
Total	1,422.3		1,416.3
Current liabilities:
Accounts payable	95.1		109.6
Intercompany loans payable	78.8		148.0
Accrued salaries, wages and related expenses	28.7		38.3
Other accrued liabilities	46.7		46.7
Total current liabilities	249.3		342.6
Long-term portion of operating lease liabilities	26.0
Net liabilities of Salaried VEBA	32.6		32.4
Deferred tax liabilities	0.0		0.0
Long-term intercompany loans payable	10.4		9.8
Long-term liabilities	59.7		63.3
Long-term debt	0.0		0.0
Total liabilities	378.0		448.1
Total stockholders' equity	1,044.3		968.2
Total	1,422.3		1,416.3
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	1.6		3.0
Short-term investments	0.0		0.0
Receivables:
Trade receivables, net	5.1		5.8
Intercompany receivables	4.5		4.3
Other	0.9		2.3
Contract assets	2.1		2.9
Inventories	14.8		12.1
Prepaid expenses and other current assets	0.6		0.4
Total current assets	29.6		30.8
Investments in and advances to subsidiaries	0.2		0.2
Property, plant and equipment, net	34.0		34.4
Operating lease assets	0.0
Long-term intercompany loans receivable	10.4		9.8
Deferred tax assets, net	0.0		0.0
Intangible assets, net	8.1		8.8
Goodwill	25.2		25.2
Other assets	0.0		0.0
Total	107.5		109.2
Current liabilities:
Accounts payable	4.5		6.9
Intercompany loans payable	0.2		0.1
Accrued salaries, wages and related expenses	1.7		1.8
Other accrued liabilities	0.8		0.8
Total current liabilities	7.2		9.6
Long-term portion of operating lease liabilities	0.0
Net liabilities of Salaried VEBA	0.0		0.0
Deferred tax liabilities	4.2		4.2
Long-term intercompany loans payable	0.0		0.0
Long-term liabilities	3.5		3.1
Long-term debt	0.0		0.0
Total liabilities	14.9		16.9
Total stockholders' equity	92.6		92.3
Total	$ 107.5		$ 109.2