Condensed Guarantor and Non-Guarantor Financial Information, Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 172.1			$ 125.6	$ 94.3
Short-term investments	25.3			36.7
Receivables:
Trade receivables, net	188.1			179.8
Intercompany receivables	0.0			0.0
Other	26.1			25.6
Contract assets	46.1			54.9
Inventories	192.5			215.1
Prepaid expenses and other current assets	18.4			18.9
Total current assets	668.6			656.6
Investments in and advances to subsidiaries	0.0			0.0
Property, plant and equipment, net	614.3			611.8
Operating lease assets	26.3			0.0
Long-term intercompany loans receivable	0.0			0.0
Deferred tax assets, net	6.3			35.9
Intangible assets, net	30.3			32.4
Goodwill	44.0			44.0
Other assets	36.5			38.6
Total	1,426.3			1,419.3
Current liabilities:
Accounts payable	93.8			121.4
Intercompany loans payable	0.0			0.0
Accrued salaries, wages and related expenses	31.9			40.1
Other accrued liabilities	50.1			44.0
Total current liabilities	175.8			205.5
Long-term portion of operating lease liabilities	25.6			0.0
Net liabilities of Salaried VEBA	32.8			32.4
Deferred tax liabilities	4.2			4.2
Long-term intercompany loans payable	0.0			0.0
Long-term liabilities	65.4			66.4
Long-term debt	371.1			370.4
Total liabilities	674.9			678.9
Total stockholders' equity	751.4	$ 746.3	$ 747.9	740.4	$ 762.5	$ 761.0	$ 753.6	$ 746.3
Total	1,426.3			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	(57.5)			(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	(57.5)			(148.1)
Investments in and advances to subsidiaries	(1,177.3)			(1,069.8)
Property, plant and equipment, net	0.0			0.0
Operating lease assets	0.0
Long-term intercompany loans receivable	(17.0)			(9.8)
Deferred tax assets, net	3.0			3.0
Intangible assets, net	0.0			0.0
Goodwill	0.0			0.0
Other assets	0.1			0.0
Total	(1,248.7)			(1,224.7)
Current liabilities:
Accounts payable	0.0			0.0
Intercompany loans payable	(57.5)			(148.1)
Accrued salaries, wages and related expenses	0.0			0.0
Other accrued liabilities	(4.8)			(6.3)
Total current liabilities	(62.3)			(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	(17.0)			(9.8)
Long-term liabilities	0.0			0.0
Long-term debt	0.0			0.0
Total liabilities	(79.3)			(164.2)
Total stockholders' equity	(1,169.4)			(1,060.5)
Total	(1,248.7)			(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	50.6			143.7
Other	0.0			0.0
Contract assets	0.0			0.0
Inventories	0.0			0.0
Prepaid expenses and other current assets	0.3			0.1
Total current assets	50.9			143.8
Investments in and advances to subsidiaries	1,081.4			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	1,132.3			1,118.5
Current liabilities:
Accounts payable	1.5			4.9
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	9.8			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	371.1			370.4
Total liabilities	380.9			378.1
Total stockholders' equity	751.4			740.4
Total	1,132.3			1,118.5
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	170.7			122.6
Short-term investments	25.3			36.7
Receivables:
Trade receivables, net	183.2			174.0
Intercompany receivables	1.4			0.1
Other	24.3			23.3
Contract assets	43.6			52.0
Inventories	183.1			203.0
Prepaid expenses and other current assets	17.7			18.4
Total current assets	649.3			630.1
Investments in and advances to subsidiaries	95.7			94.9
Property, plant and equipment, net	580.6			577.4
Operating lease assets	26.3
Long-term intercompany loans receivable	0.0			0.0
Deferred tax assets, net	3.3			32.9
Intangible assets, net	22.6			23.6
Goodwill	18.7			18.8
Other assets	36.2			38.6
Total	1,432.7			1,416.3
Current liabilities:
Accounts payable	86.7			109.6
Intercompany loans payable	56.1			148.0
Accrued salaries, wages and related expenses	30.3			38.3
Other accrued liabilities	46.1			46.7
Total current liabilities	219.2			342.6
Long-term portion of operating lease liabilities	25.6
Net liabilities of Salaried VEBA	32.8			32.4
Deferred tax liabilities	0.0			0.0
Long-term intercompany loans payable	17.0			9.8
Long-term liabilities	61.8			63.3
Long-term debt	0.0			0.0
Total liabilities	356.4			448.1
Total stockholders' equity	1,076.3			968.2
Total	1,432.7			1,416.3
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	1.4			3.0
Short-term investments	0.0			0.0
Receivables:
Trade receivables, net	4.9			5.8
Intercompany receivables	5.5			4.3
Other	1.8			2.3
Contract assets	2.5			2.9
Inventories	9.4			12.1
Prepaid expenses and other current assets	0.4			0.4
Total current assets	25.9			30.8
Investments in and advances to subsidiaries	0.2			0.2
Property, plant and equipment, net	33.7			34.4
Operating lease assets	0.0
Long-term intercompany loans receivable	17.0			9.8
Deferred tax assets, net	0.0			0.0
Intangible assets, net	7.7			8.8
Goodwill	25.3			25.2
Other assets	0.2			0.0
Total	110.0			109.2
Current liabilities:
Accounts payable	5.6			6.9
Intercompany loans payable	1.4			0.1
Accrued salaries, wages and related expenses	1.6			1.8
Other accrued liabilities	0.5			0.8
Total current liabilities	9.1			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	3.6			3.1
Long-term debt	0.0			0.0
Total liabilities	16.9			16.9
Total stockholders' equity	93.1			92.3
Total	$ 110.0			$ 109.2