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Divestitures - Narrative (Details) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2020 USD ($)	Jul. 22, 2019 USD ($)	Jun. 03, 2019 USD ($) facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of facilities sold | facility									2
Goodwill	$ 240.7			$ 240.7	$ 525.8
Gain (loss) on disposal				0.0	(1.8)	$ 0.0
Non-cash restructuring charges	1,041.5			1,041.5	0.0	0.0
Forged Products
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration received									$ 37.0
Proceeds from sale of businesses, net of transaction costs					33.0
Goodwill	$ 173.4			$ 173.4			$ 460.4		$ 10.4
Gain (loss) on disposal			$ (7.7)		(8.1)
Prior year revenue from facilities sold						86.0
Cast Products
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration received								$ 127.0
Proceeds from sale of businesses, net of transaction costs					125.1
Gain (loss) on disposal		$ 6.2			6.2
Prior year revenue from facilities sold						$ 105.0
Non-cash restructuring charges					10.2
Property, Plant and Equipment | Cast Products
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-cash restructuring charges					4.5
Right of use assets, operating leases [Member] | Cast Products
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-cash restructuring charges					1.4
Right of use assets, finance leases [Member] | Cast Products
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-cash restructuring charges					1.0
Finite-Lived Intangible Assets [Member] | Cast Products
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-cash restructuring charges					$ 3.3

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