Business Segments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Sales	$ 1,009.5	$ 880.2	$ 1,988.5	$ 1,746.1
Operating profit (loss)	124.0	70.9	220.4	140.8
LIFO and net realizable value reserves	0.0	(0.1)	0.0	(0.1)
Corporate expenses	(12.9)	(11.8)	(26.1)	(22.1)
Closed operations and other expenses	(5.1)	(13.2)	(13.2)	(16.2)
Gain on joint venture deconsolidation (See Note 5)	0.0	0.0	15.9	0.0
Interest expense, net	(25.5)	(34.5)	(51.0)	(68.0)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	80.5	11.3	146.0	34.4
Operating Segments
Segment Reporting Information [Line Items]
Sales	1,051.0	916.5	2,069.5	1,813.2
Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	611.7	543.3	1,191.1	1,067.0
Operating profit (loss)	97.9	68.0	183.4	118.9
Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	439.3	373.2	878.4	746.2
Operating profit (loss)	26.1	2.9	37.0	21.9
Internal Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	41.5	36.3	81.0	67.1
Internal Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	19.8	16.9	38.5	30.2
Internal Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	21.7	19.4	42.5	36.9
External Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	1,009.5	880.2	1,988.5	1,746.1
External Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	591.9	526.4	1,152.6	1,036.8
External Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	$ 417.6	$ 353.8	$ 835.9	$ 709.3

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