Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue		$ 770.3	$ 1,080.4	$ 1,725.8	$ 2,085.2
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		83.00%	86.00%	84.00%	86.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		30.00%	33.00%	31.00%	32.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		14.00%	19.00%	15.00%	19.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	18.00%	18.00%	18.00%
Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		12.00%	10.00%	12.00%	11.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		8.00%	6.00%	8.00%	6.00%
Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		17.00%	14.00%	16.00%	14.00%
United States
Disaggregation of Revenue [Line Items]
Revenue		$ 477.2	$ 640.5	$ 1,083.3	$ 1,261.6
Europe
Disaggregation of Revenue [Line Items]
Revenue		124.4	207.2	295.5	418.2
Asia
Disaggregation of Revenue [Line Items]
Revenue		118.7	171.9	238.0	295.1
Canada
Disaggregation of Revenue [Line Items]
Revenue		16.2	29.4	38.2	55.8
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		33.8	31.4	70.8	54.5
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		365.6	572.2	858.1	1,097.8
Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	167.5	213.4	338.1	381.9
Automotive
Disaggregation of Revenue [Line Items]
Revenue		50.8	73.6	127.3	150.5
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		34.8	52.6	78.4	110.5
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		47.2	49.6	97.6	102.8
Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		39.5	38.1	72.7	72.2
Medical
Disaggregation of Revenue [Line Items]
Revenue		29.9	42.4	68.5	88.5
Other
Disaggregation of Revenue [Line Items]
Revenue		35.0	38.5	85.1	81.0
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		822.9	1,165.6	1,859.0	2,254.1
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		$ 316.7	$ 552.8	$ 765.2	$ 1,069.1
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Operating Segments | High Performance Materials & Components | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		36.00%	37.00%	38.00%	37.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		35.00%	38.00%	35.00%	38.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		29.00%	25.00%	27.00%	25.00%
Operating Segments | High Performance Materials & Components | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 166.2	$ 290.2	$ 393.5	$ 559.2
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue		92.0	160.9	225.5	319.5
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue		27.6	49.4	52.7	92.1
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue		5.1	14.9	16.7	26.3
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		9.8	17.7	32.6	32.6
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		252.3	444.8	611.1	845.4
Operating Segments | High Performance Materials & Components | Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	22.8	33.4	46.9	61.5
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue		1.0	3.1	3.0	6.7
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		5.0	13.4	10.3	31.9
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		0.1	0.0	0.1	0.1
Operating Segments | High Performance Materials & Components | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		0.2	0.3	0.5	0.3
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue		10.3	21.1	27.3	45.8
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue		9.0	17.0	21.8	38.0
Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 506.2	$ 612.8	$ 1,093.8	$ 1,185.0
Percent of revenue		100.00%	100.00%	100.00%	100.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue		72.00%	72.00%	71.00%	71.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		26.00%	30.00%	25.00%	26.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue		0.00%	0.00%	0.00%	0.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		13.00%	10.00%	12.00%	11.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Precision rolled strip products
Disaggregation of Revenue [Line Items]
Percent of revenue		19.00%	21.00%	21.00%	22.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue		14.00%	11.00%	13.00%	12.00%
Operating Segments | Advanced Alloys & Solutions [Member] | Standard stainless products [Member]
Disaggregation of Revenue [Line Items]
Percent of revenue		28.00%	28.00%	29.00%	29.00%
Operating Segments | Advanced Alloys & Solutions [Member] | United States
Disaggregation of Revenue [Line Items]
Revenue		$ 311.0	$ 350.3	$ 689.8	$ 702.4
Operating Segments | Advanced Alloys & Solutions [Member] | Europe
Disaggregation of Revenue [Line Items]
Revenue		32.4	46.3	70.0	98.7
Operating Segments | Advanced Alloys & Solutions [Member] | Asia
Disaggregation of Revenue [Line Items]
Revenue		91.1	122.5	185.3	203.0
Operating Segments | Advanced Alloys & Solutions [Member] | Canada
Disaggregation of Revenue [Line Items]
Revenue		11.1	14.5	21.5	29.5
Operating Segments | Advanced Alloys & Solutions [Member] | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue		24.0	13.7	38.2	21.9
Operating Segments | Advanced Alloys & Solutions [Member] | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue		113.3	127.4	247.0	252.4
Operating Segments | Advanced Alloys & Solutions [Member] | Energy*
Disaggregation of Revenue [Line Items]
Revenue	[1]	144.7	180.0	291.2	320.4
Operating Segments | Advanced Alloys & Solutions [Member] | Automotive
Disaggregation of Revenue [Line Items]
Revenue		49.8	70.5	124.3	143.8
Operating Segments | Advanced Alloys & Solutions [Member] | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue		29.8	39.2	68.1	78.6
Operating Segments | Advanced Alloys & Solutions [Member] | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue		47.1	49.6	97.5	102.7
Operating Segments | Advanced Alloys & Solutions [Member] | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue		39.3	37.8	72.2	71.9
Operating Segments | Advanced Alloys & Solutions [Member] | Medical
Disaggregation of Revenue [Line Items]
Revenue		19.6	21.3	41.2	42.7
Operating Segments | Advanced Alloys & Solutions [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue		26.0	21.5	63.3	43.0
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		770.3	1,080.4	1,725.8	2,085.2
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue		300.7	533.1	721.0	1,029.7
External Customers | Operating Segments | Advanced Alloys & Solutions [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 469.6	$ 547.3	$ 1,004.8	$ 1,055.5

[1] Includes the oil & gas, hydrocarbon and chemical processing, and electrical energy markets.