Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 1,009.5	$ 880.2	$ 1,988.5	$ 1,746.1
Percent of total revenue	100.00%	100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of total revenue	84.00%	82.00%	84.00%	82.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	30.00%	27.00%	30.00%	26.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of total revenue	20.00%	19.00%	20.00%	18.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	16.00%	16.00%	16.00%	17.00%
Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of total revenue	13.00%	13.00%	13.00%	14.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	5.00%	7.00%	5.00%	7.00%
Total Standard Products
Disaggregation of Revenue [Line Items]
Percent of total revenue	16.00%	18.00%	16.00%	18.00%
Stainless steel sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue	9.00%	9.00%	9.00%	9.00%
Specialty stainless sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue	4.00%	5.00%	4.00%	5.00%
Stainless steel plate and other
Disaggregation of Revenue [Line Items]
Percent of total revenue	3.00%	4.00%	3.00%	4.00%
United States
Disaggregation of Revenue [Line Items]
Revenue	$ 575.8	$ 524.2	$ 1,130.1	$ 1,049.2
Europe
Disaggregation of Revenue [Line Items]
Revenue	230.5	201.6	456.0	388.0
Asia
Disaggregation of Revenue [Line Items]
Revenue	143.7	94.5	294.5	197.2
Canada
Disaggregation of Revenue [Line Items]
Revenue	29.3	28.5	56.5	52.4
South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue	30.2	31.4	51.4	59.3
Total Key Markets
Disaggregation of Revenue [Line Items]
Revenue	813.2	695.0	1,604.5	1,382.8
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	481.9	435.6	944.4	852.8
Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue	132.7	98.5	285.3	191.4
Automotive
Disaggregation of Revenue [Line Items]
Revenue	80.4	68.8	159.5	144.7
Electrical Energy
Disaggregation of Revenue [Line Items]
Revenue	68.2	44.1	120.4	95.7
Medical
Disaggregation of Revenue [Line Items]
Revenue	50.0	48.0	94.9	98.2
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue	63.6	55.4	122.5	114.4
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	55.9	46.3	111.5	96.3
Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue	35.5	31.5	68.4	66.0
Other
Disaggregation of Revenue [Line Items]
Revenue	41.3	52.0	81.6	86.6
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,051.0	916.5	2,069.5	1,813.2
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	$ 611.7	$ 543.3	$ 1,191.1	$ 1,067.0
Percent of total 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 total 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 total revenue	32.00%	31.00%	31.00%	31.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of total revenue	35.00%	33.00%	36.00%	32.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	23.00%	25.00%	24.00%	26.00%
Operating Segments | High Performance Materials & Components | Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of total 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 total revenue	10.00%	11.00%	9.00%	11.00%
Operating Segments | High Performance Materials & Components | Total Standard Products
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Stainless steel sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Specialty stainless sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Stainless steel plate and other
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | United States
Disaggregation of Revenue [Line Items]
Revenue	$ 292.1	$ 271.9	$ 581.9	$ 547.1
Operating Segments | High Performance Materials & Components | Europe
Disaggregation of Revenue [Line Items]
Revenue	195.2	176.4	392.2	337.4
Operating Segments | High Performance Materials & Components | Asia
Disaggregation of Revenue [Line Items]
Revenue	71.5	40.1	119.8	82.2
Operating Segments | High Performance Materials & Components | Canada
Disaggregation of Revenue [Line Items]
Revenue	18.4	20.0	35.0	37.1
Operating Segments | High Performance Materials & Components | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue	14.7	18.0	23.7	33.0
Operating Segments | High Performance Materials & Components | Total Key Markets
Disaggregation of Revenue [Line Items]
Revenue	545.8	489.2	1,062.3	965.7
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	438.5	398.8	865.2	780.2
Operating Segments | High Performance Materials & Components | Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue	18.3	18.9	33.5	35.4
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue	2.8	2.0	5.4	4.0
Operating Segments | High Performance Materials & Components | Electrical Energy
Disaggregation of Revenue [Line Items]
Revenue	40.2	24.1	71.0	53.7
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue	46.0	45.4	87.2	92.4
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.5	0.1	0.8
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	19.0	12.5	36.6	24.1
Operating Segments | High Performance Materials & Components | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue	2.3	1.2	3.7	2.4
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue	24.8	23.0	49.9	43.8
Operating Segments | Flat Rolled Products
Disaggregation of Revenue [Line Items]
Revenue	$ 439.3	$ 373.2	$ 878.4	$ 746.2
Percent of total 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 total revenue	64.00%	60.00%	66.00%	60.00%
Operating Segments | Flat Rolled Products | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	27.00%	22.00%	29.00%	21.00%
Operating Segments | Flat Rolled Products | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of total 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 total revenue	5.00%	5.00%	5.00%	5.00%
Operating Segments | Flat Rolled Products | Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of total revenue	32.00%	33.00%	32.00%	34.00%
Operating Segments | Flat Rolled Products | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue	0.00%	0.00%	0.00%	0.00%
Operating Segments | Flat Rolled Products | Total Standard Products
Disaggregation of Revenue [Line Items]
Percent of total revenue	36.00%	40.00%	34.00%	40.00%
Operating Segments | Flat Rolled Products | Stainless steel sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue	21.00%	22.00%	20.00%	23.00%
Operating Segments | Flat Rolled Products | Specialty stainless sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue	11.00%	13.00%	10.00%	12.00%
Operating Segments | Flat Rolled Products | Stainless steel plate and other
Disaggregation of Revenue [Line Items]
Percent of total revenue	4.00%	5.00%	4.00%	5.00%
Operating Segments | Flat Rolled Products | United States
Disaggregation of Revenue [Line Items]
Revenue	$ 283.7	$ 252.3	$ 548.2	$ 502.1
Operating Segments | Flat Rolled Products | Europe
Disaggregation of Revenue [Line Items]
Revenue	35.3	25.2	63.8	50.6
Operating Segments | Flat Rolled Products | Asia
Disaggregation of Revenue [Line Items]
Revenue	72.2	54.4	174.7	115.0
Operating Segments | Flat Rolled Products | Canada
Disaggregation of Revenue [Line Items]
Revenue	10.9	8.5	21.5	15.3
Operating Segments | Flat Rolled Products | South America, Middle East and other
Disaggregation of Revenue [Line Items]
Revenue	15.5	13.4	27.7	26.3
Operating Segments | Flat Rolled Products | Total Key Markets
Disaggregation of Revenue [Line Items]
Revenue	267.4	205.8	542.2	417.1
Operating Segments | Flat Rolled Products | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue	43.4	36.8	79.2	72.6
Operating Segments | Flat Rolled Products | Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue	114.4	79.6	251.8	156.0
Operating Segments | Flat Rolled Products | Automotive
Disaggregation of Revenue [Line Items]
Revenue	77.6	66.8	154.1	140.7
Operating Segments | Flat Rolled Products | Electrical Energy
Disaggregation of Revenue [Line Items]
Revenue	28.0	20.0	49.4	42.0
Operating Segments | Flat Rolled Products | Medical
Disaggregation of Revenue [Line Items]
Revenue	4.0	2.6	7.7	5.8
Operating Segments | Flat Rolled Products | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue	63.6	54.9	122.4	113.6
Operating Segments | Flat Rolled Products | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue	36.9	33.8	74.9	72.2
Operating Segments | Flat Rolled Products | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue	33.2	30.3	64.7	63.6
Operating Segments | Flat Rolled Products | Other
Disaggregation of Revenue [Line Items]
Revenue	16.5	29.0	31.7	42.8
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,009.5	880.2	1,988.5	1,746.1
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue	591.9	526.4	1,152.6	1,036.8
External Customers | Operating Segments | Flat Rolled Products
Disaggregation of Revenue [Line Items]
Revenue	$ 417.6	$ 353.8	$ 835.9	$ 709.3