Condensed Guarantor and Non-Guarantor Financial Information, Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 115.3	$ 125.6	$ 135.0
Short-term investments	22.3	36.7
Receivables:
Trade receivables, net	195.8	179.8
Intercompany receivables	0.0	0.0
Other	22.4	25.6
Contract assets	58.4	54.9
Inventories	233.9	215.1
Prepaid expenses and other current assets	23.0	18.9
Total current assets	671.1	656.6
Investments in and advances to subsidiaries	0.0	0.0
Property, plant and equipment, net	610.9	611.8
Operating lease assets	27.4	0.0
Long-term intercompany loans receivable	0.0	0.0
Deferred tax assets, net	22.4	35.9
Intangible assets, net	31.7	32.4
Goodwill	44.0	44.0
Other assets	40.3	38.6
Total	1,447.8	1,419.3
Current liabilities:
Accounts payable	124.4	121.4
Intercompany loans payable	0.0	0.0
Accrued salaries, wages and related expenses	28.2	40.1
Other accrued liabilities	51.6	44.0
Total current liabilities	204.2	205.5
Long-term portion of operating lease liabilities	26.6	0.0
Net liabilities of Salaried VEBA	32.5	32.4
Deferred tax liabilities	4.2	4.2
Long-term intercompany loans payable	0.0	0.0
Long-term liabilities	61.8	66.4
Long-term debt	370.6	370.4
Total liabilities	699.9	678.9
Total stockholders' equity	747.9	740.4	$ 753.6	$ 746.3
Total	1,447.8	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	(111.4)	(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	(111.4)	(148.1)
Investments in and advances to subsidiaries	(1,117.0)	(1,069.8)
Property, plant and equipment, net	0.0	0.0
Operating lease assets	0.0
Long-term intercompany loans receivable	(7.2)	(9.8)
Deferred tax assets, net	3.0	3.0
Intangible assets, net	0.0	0.0
Goodwill	0.0	0.0
Other assets	0.0	0.0
Total	(1,232.6)	(1,224.7)
Current liabilities:
Accounts payable	0.0	0.0
Intercompany loans payable	(111.4)	(148.1)
Accrued salaries, wages and related expenses	0.0	0.0
Other accrued liabilities	(1.5)	(6.3)
Total current liabilities	(112.9)	(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	(7.2)	(9.8)
Long-term liabilities	0.0	0.0
Long-term debt	0.0	0.0
Total liabilities	(120.1)	(164.2)
Total stockholders' equity	(1,112.5)	(1,060.5)
Total	(1,232.6)	(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	106.7	143.7
Other	0.0	0.0
Contract assets	0.0	0.0
Inventories	0.0	0.0
Prepaid expenses and other current assets	0.1	0.1
Total current assets	106.8	143.8
Investments in and advances to subsidiaries	1,021.3	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	1,128.1	1,118.5
Current liabilities:
Accounts payable	1.3	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	9.6	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	370.6	370.4
Total liabilities	380.2	378.1
Total stockholders' equity	747.9	740.4
Total	1,128.1	1,118.5
Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	111.2	122.6
Short-term investments	22.3	36.7
Receivables:
Trade receivables, net	190.3	174.0
Intercompany receivables	0.3	0.1
Other	20.8	23.3
Contract assets	53.8	52.0
Inventories	220.0	203.0
Prepaid expenses and other current assets	22.5	18.4
Total current assets	641.2	630.1
Investments in and advances to subsidiaries	95.5	94.9
Property, plant and equipment, net	576.9	577.4
Operating lease assets	27.4
Long-term intercompany loans receivable	0.0	0.0
Deferred tax assets, net	19.4	32.9
Intangible assets, net	23.2	23.6
Goodwill	18.7	18.8
Other assets	40.3	38.6
Total	1,442.6	1,416.3
Current liabilities:
Accounts payable	115.6	109.6
Intercompany loans payable	111.3	148.0
Accrued salaries, wages and related expenses	26.5	38.3
Other accrued liabilities	44.4	46.7
Total current liabilities	297.8	342.6
Long-term portion of operating lease liabilities	26.6
Net liabilities of Salaried VEBA	32.5	32.4
Deferred tax liabilities	0.0	0.0
Long-term intercompany loans payable	7.2	9.8
Long-term liabilities	58.8	63.3
Long-term debt	0.0	0.0
Total liabilities	422.9	448.1
Total stockholders' equity	1,019.7	968.2
Total	1,442.6	1,416.3
Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	4.1	3.0
Short-term investments	0.0	0.0
Receivables:
Trade receivables, net	5.5	5.8
Intercompany receivables	4.4	4.3
Other	1.6	2.3
Contract assets	4.6	2.9
Inventories	13.9	12.1
Prepaid expenses and other current assets	0.4	0.4
Total current assets	34.5	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	7.2	9.8
Deferred tax assets, net	0.0	0.0
Intangible assets, net	8.5	8.8
Goodwill	25.3	25.2
Other assets	0.0	0.0
Total	109.7	109.2
Current liabilities:
Accounts payable	7.5	6.9
Intercompany loans payable	0.1	0.1
Accrued salaries, wages and related expenses	1.7	1.8
Other accrued liabilities	0.4	0.8
Total current liabilities	9.7	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	3.0	3.1
Long-term debt	0.0	0.0
Total liabilities	16.9	16.9
Total stockholders' equity	92.8	92.3
Total	$ 109.7	$ 109.2