Business Segments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Sales	$ 1,080.4	$ 1,009.5	$ 2,085.2	$ 1,988.5
Operating profit (loss)	114.5	124.0	176.2	220.4
LIFO and net realizable value reserves	0.0	0.0	(0.1)	0.0
Corporate expenses	(18.0)	(12.9)	(34.6)	(26.1)
Closed operations and other expenses	(7.9)	(5.1)	(11.0)	(13.2)
Gain on joint venture deconsolidation	0.0	0.0	0.0	15.9
Gain on asset sales, net	21.6	0.0	21.6	0.0
Interest expense, net	(25.9)	(25.5)	(50.7)	(51.0)
Income from Continuing Operations before Income Taxes, Noncontrolling Interest	84.3	80.5	101.4	146.0
Operating Segments
Segment Reporting Information [Line Items]
Sales	1,135.6	1,051.0	2,186.7	2,069.5
Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	673.8	611.7	1,297.7	1,191.1
Operating profit (loss)	98.9	97.9	171.5	183.4
Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	461.8	439.3	889.0	878.4
Operating profit (loss)	15.6	26.1	4.7	37.0
Internal Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	55.2	41.5	101.5	81.0
Internal Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	31.4	19.8	54.1	38.5
Internal Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	23.8	21.7	47.4	42.5
External Customers \| Operating Segments
Segment Reporting Information [Line Items]
Sales	1,080.4	1,009.5	2,085.2	1,988.5
External Customers \| Operating Segments \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Sales	642.4	591.9	1,243.6	1,152.6
External Customers \| Operating Segments \| Flat Rolled Products
Segment Reporting Information [Line Items]
Sales	$ 438.0	$ 417.6	$ 841.6	$ 835.9

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