Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 1,080.4	$ 1,009.5	$ 2,085.2	$ 1,988.5
Percent of revenue	100.00%	100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue	88.00%	85.00%	87.00%	85.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue	33.00%	30.00%	31.00%	30.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue	19.00%	20.00%	19.00%	21.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue	18.00%	16.00%	18.00%	16.00%
Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of revenue	12.00%	13.00%	13.00%	13.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue	6.00%	6.00%	6.00%	5.00%
Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue	12.00%	15.00%	13.00%	15.00%
United States
Disaggregation of Revenue [Line Items]
Revenue	$ 640.5	$ 575.8	$ 1,261.6	$ 1,130.1
Europe
Disaggregation of Revenue [Line Items]
Revenue	207.2	230.5	418.2	456.0
Asia
Disaggregation of Revenue [Line Items]
Revenue	171.9	143.7	295.1	294.5
Canada
Disaggregation of Revenue [Line Items]
Revenue	29.4	29.3	55.8	56.5
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue	31.4	30.2	54.5	51.4
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	572.2	481.9	1,097.8	944.4
Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue	136.0	132.7	248.8	285.3
Energy
Disaggregation of Revenue [Line Items]
Revenue	77.4	68.2	133.1	120.4
Automotive
Disaggregation of Revenue [Line Items]
Revenue	73.6	80.4	150.5	159.5
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	52.6	55.9	110.5	111.5
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue	49.6	63.6	102.8	122.5
Medical
Disaggregation of Revenue [Line Items]
Revenue	42.4	50.0	88.5	94.9
Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue	38.1	35.5	72.2	68.4
Other
Disaggregation of Revenue [Line Items]
Revenue	38.5	41.3	81.0	81.6
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,135.6	1,051.0	2,186.7	2,069.5
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	$ 673.8	$ 611.7	$ 1,297.7	$ 1,191.1
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	31.00%	32.00%	31.00%	31.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue	32.00%	35.00%	32.00%	36.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue	27.00%	23.00%	27.00%	24.00%
Operating Segments | High Performance Materials & Components | Precision and engineered strip
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	10.00%	10.00%	10.00%	9.00%
Operating Segments | High Performance Materials & Components | Standard stainless products
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	$ 357.6	$ 292.1	$ 693.6	$ 581.9
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue	181.1	195.2	359.4	392.2
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue	60.5	71.5	113.3	119.8
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue	24.5	18.4	43.6	35.0
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue	18.7	14.7	33.7	23.7
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	512.3	438.5	977.4	865.2
Operating Segments | High Performance Materials & Components | Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue	18.4	18.3	35.1	33.5
Operating Segments | High Performance Materials & Components | Energy
Disaggregation of Revenue [Line Items]
Revenue	34.1	40.2	61.4	71.0
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue	3.1	2.8	6.7	5.4
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	13.4	19.0	31.9	36.6
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.1	0.1
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue	38.6	46.0	81.3	87.2
Operating Segments | High Performance Materials & Components | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue	1.2	2.3	2.3	3.7
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue	21.3	24.8	47.4	49.9
Operating Segments | Flat Rolled Products
Disaggregation of Revenue [Line Items]
Revenue	$ 461.8	$ 439.3	$ 889.0	$ 878.4
Percent of revenue	100.00%	100.00%	100.00%	100.00%
Operating Segments | Flat Rolled Products | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of revenue	69.00%	64.00%	69.00%	66.00%
Operating Segments | Flat Rolled Products | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of revenue	35.00%	27.00%	32.00%	29.00%
Operating Segments | Flat Rolled Products | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | Flat Rolled Products | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of revenue	4.00%	5.00%	6.00%	5.00%
Operating Segments | Flat Rolled Products | Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of revenue	30.00%	32.00%	31.00%	32.00%
Operating Segments | Flat Rolled Products | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | Flat Rolled Products | Standard stainless products
Disaggregation of Revenue [Line Items]
Percent of revenue	31.00%	36.00%	31.00%	34.00%
Operating Segments | Flat Rolled Products | United States
Disaggregation of Revenue [Line Items]
Revenue	$ 282.9	$ 283.7	$ 568.0	$ 548.2
Operating Segments | Flat Rolled Products | Europe
Disaggregation of Revenue [Line Items]
Revenue	26.1	35.3	58.8	63.8
Operating Segments | Flat Rolled Products | Asia
Disaggregation of Revenue [Line Items]
Revenue	111.4	72.2	181.8	174.7
Operating Segments | Flat Rolled Products | Canada
Disaggregation of Revenue [Line Items]
Revenue	4.9	10.9	12.2	21.5
Operating Segments | Flat Rolled Products | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue	12.7	15.5	20.8	27.7
Operating Segments | Flat Rolled Products | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	59.9	43.4	120.4	79.2
Operating Segments | Flat Rolled Products | Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue	117.6	114.4	213.7	251.8
Operating Segments | Flat Rolled Products | Energy
Disaggregation of Revenue [Line Items]
Revenue	43.3	28.0	71.7	49.4
Operating Segments | Flat Rolled Products | Automotive
Disaggregation of Revenue [Line Items]
Revenue	70.5	77.6	143.8	154.1
Operating Segments | Flat Rolled Products | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	39.2	36.9	78.6	74.9
Operating Segments | Flat Rolled Products | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue	49.6	63.6	102.7	122.4
Operating Segments | Flat Rolled Products | Medical
Disaggregation of Revenue [Line Items]
Revenue	3.8	4.0	7.2	7.7
Operating Segments | Flat Rolled Products | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue	36.9	33.2	69.9	64.7
Operating Segments | Flat Rolled Products | Other
Disaggregation of Revenue [Line Items]
Revenue	17.2	16.5	33.6	31.7
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,080.4	1,009.5	2,085.2	1,988.5
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	642.4	591.9	1,243.6	1,152.6
External Customers | Operating Segments | Flat Rolled Products
Disaggregation of Revenue [Line Items]
Revenue	$ 438.0	$ 417.6	$ 841.6	$ 835.9