Business Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Sales	$ 796.1	$ 770.5	$ 810.5	$ 757.5	$ 738.9	$ 832.7	$ 1,022.5	$ 1,125.5	$ 3,134.6	$ 3,719.6	$ 4,223.4
External Sales [Member]
Segment Reporting Information [Line Items]
Sales									3,134.6	3,719.6	4,223.4
Operating Segments [Member]
Segment Reporting Information [Line Items]
Sales									3,240.3	3,870.6	4,404.8
Operating Segments [Member] | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									105.7	151.0	181.4
Operating Segments [Member] | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									3,134.6	3,719.6	4,223.4
Operating Segments [Member] | High Performance Materials & Components [Member]
Segment Reporting Information [Line Items]
Sales									1,979.5	2,062.7	2,084.6
Operating Segments [Member] | High Performance Materials & Components [Member] | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									49.1	76.8	77.8
Operating Segments [Member] | High Performance Materials & Components [Member] | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									1,930.4	1,985.9	2,006.8
Operating Segments [Member] | Flat Rolled Products [Member]
Segment Reporting Information [Line Items]
Sales									1,260.8	1,807.9	2,320.2
Operating Segments [Member] | Flat Rolled Products [Member] | Intersegment Sales [Member]
Segment Reporting Information [Line Items]
Sales									56.6	74.2	103.6
Operating Segments [Member] | Flat Rolled Products [Member] | External Sales [Member]
Segment Reporting Information [Line Items]
Sales									$ 1,204.2	$ 1,733.7	$ 2,216.6