Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Sales	$ 1,004.8	$ 979.0
Operating profit (loss)	61.7	96.4
LIFO and net realizable value reserves	(0.1)	0.0
Corporate expenses	(16.6)	(13.2)
Closed operations and other expenses	(3.1)	(8.1)
Gain on joint venture deconsolidation (See Note 5)	0.0	15.9
Interest expense, net	(24.8)	(25.5)
Income from Continuing Operations before Income Taxes, Noncontrolling Interest	17.1	65.5
Operating Segments
Segment Reporting Information [Line Items]
Sales	1,051.1	1,018.5
Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	623.9	579.4
Operating profit (loss)	72.6	85.5
Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	427.2	439.1
Operating profit (loss)	(10.9)	10.9
Internal Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	46.3	39.5
Internal Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	22.7	18.7
Internal Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	23.6	20.8
External Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	1,004.8	979.0
External Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	601.2	560.7
External Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	$ 403.6	$ 418.3

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