Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue	$ 1,037.9	$ 1,020.2	$ 1,009.5	$ 979.0	$ 909.9	$ 869.1	$ 880.2	$ 865.9	$ 4,046.6	$ 3,525.1	$ 3,134.6
Percent of total revenue									100.00%	100.00%	100.00%
Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of total revenue									86.00%	84.00%	87.00%
Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									30.00%	28.00%	28.00%
Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of total revenue									20.00%	19.00%	18.00%
Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									17.00%	17.00%	20.00%
Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of total revenue									14.00%	14.00%	13.00%
Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									5.00%	6.00%	8.00%
Total Standard Products
Disaggregation of Revenue [Line Items]
Percent of total revenue									14.00%	16.00%	13.00%
Stainless steel sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue									8.00%	9.00%	7.00%
Specialty stainless sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue									4.00%	5.00%	4.00%
Stainless steel plate and other
Disaggregation of Revenue [Line Items]
Percent of total revenue									2.00%	2.00%	2.00%
United States
Disaggregation of Revenue [Line Items]
Revenue									$ 2,348.1	$ 2,070.6	$ 1,857.5
China
Disaggregation of Revenue [Line Items]
Revenue									320.0	265.6	214.1
Germany
Disaggregation of Revenue [Line Items]
Revenue									247.2	217.1	177.7
United Kingdom
Disaggregation of Revenue [Line Items]
Revenue									242.1	231.6	183.8
Japan
Disaggregation of Revenue [Line Items]
Revenue									214.9	131.7	151.9
France
Disaggregation of Revenue [Line Items]
Revenue									183.6	165.6	142.6
Rest of World
Disaggregation of Revenue [Line Items]
Revenue									490.7	442.9	407.0
Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									1,965.5	1,718.1	1,590.4
Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue									546.2	418.2	280.8
Automotive
Disaggregation of Revenue [Line Items]
Revenue									323.4	273.7	232.8
Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									244.9	226.0	172.2
Electrical Energy
Disaggregation of Revenue [Line Items]
Revenue									234.5	192.2	232.6
Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									226.0	192.9	160.6
Medical
Disaggregation of Revenue [Line Items]
Revenue									183.1	183.0	195.8
Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue									156.9	151.6	109.7
Other
Disaggregation of Revenue [Line Items]
Revenue									166.1	169.4	159.7
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue									4,212.6	3,654.5	3,240.3
Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue									$ 2,413.6	$ 2,127.0	$ 1,979.5
Percent of total revenue									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%
Operating Segments | High Performance Materials & Components | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									31.00%	31.00%	29.00%
Operating Segments | High Performance Materials & Components | Precision forgings, castings and components
Disaggregation of Revenue [Line Items]
Percent of total revenue									34.00%	32.00%	29.00%
Operating Segments | High Performance Materials & Components | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									25.00%	26.00%	29.00%
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%
Operating Segments | High Performance Materials & Components | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									10.00%	11.00%	13.00%
Operating Segments | High Performance Materials & Components | Total Standard Products
Disaggregation of Revenue [Line Items]
Percent of total revenue									0.00%	0.00%	0.00%
Operating Segments | High Performance Materials & Components | Stainless steel sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue									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%
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%
Operating Segments | High Performance Materials & Components | United States
Disaggregation of Revenue [Line Items]
Revenue									$ 1,214.1	$ 1,096.5	$ 1,057.3
Operating Segments | High Performance Materials & Components | China
Disaggregation of Revenue [Line Items]
Revenue									83.1	51.1	57.8
Operating Segments | High Performance Materials & Components | Germany
Disaggregation of Revenue [Line Items]
Revenue									192.7	170.5	137.4
Operating Segments | High Performance Materials & Components | United Kingdom
Disaggregation of Revenue [Line Items]
Revenue									232.4	220.9	171.1
Operating Segments | High Performance Materials & Components | Japan
Disaggregation of Revenue [Line Items]
Revenue									136.5	95.2	113.4
Operating Segments | High Performance Materials & Components | France
Disaggregation of Revenue [Line Items]
Revenue									172.7	157.8	134.9
Operating Segments | High Performance Materials & Components | Rest of World
Disaggregation of Revenue [Line Items]
Revenue									302.7	275.4	258.5
Operating Segments | High Performance Materials & Components | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									1,771.3	1,568.9	1,439.2
Operating Segments | High Performance Materials & Components | Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue									74.9	63.9	46.5
Operating Segments | High Performance Materials & Components | Automotive
Disaggregation of Revenue [Line Items]
Revenue									9.5	8.8	7.6
Operating Segments | High Performance Materials & Components | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									0.4	1.1	1.8
Operating Segments | High Performance Materials & Components | Electrical Energy
Disaggregation of Revenue [Line Items]
Revenue									131.4	113.1	129.1
Operating Segments | High Performance Materials & Components | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									72.8	51.1	36.7
Operating Segments | High Performance Materials & Components | Medical
Disaggregation of Revenue [Line Items]
Revenue									168.5	170.4	185.3
Operating Segments | High Performance Materials & Components | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue									7.9	4.4	3.4
Operating Segments | High Performance Materials & Components | Other
Disaggregation of Revenue [Line Items]
Revenue									97.5	85.7	80.8
Operating Segments | Flat Rolled Products
Disaggregation of Revenue [Line Items]
Revenue									$ 1,799.0	$ 1,527.5	$ 1,260.8
Percent of total revenue									100.00%	100.00%	100.00%
Operating Segments | Flat Rolled Products | Total High-Value Products
Disaggregation of Revenue [Line Items]
Percent of total revenue									66.00%	63.00%	65.00%
Operating Segments | Flat Rolled Products | Nickel-based alloys and specialty alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									28.00%	24.00%	25.00%
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%
Operating Segments | Flat Rolled Products | Titanium and titanium-based alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									5.00%	5.00%	4.00%
Operating Segments | Flat Rolled Products | Precision and engineered strip
Disaggregation of Revenue [Line Items]
Percent of total revenue									33.00%	34.00%	36.00%
Operating Segments | Flat Rolled Products | Zirconium and related alloys
Disaggregation of Revenue [Line Items]
Percent of total revenue									0.00%	0.00%	0.00%
Operating Segments | Flat Rolled Products | Total Standard Products
Disaggregation of Revenue [Line Items]
Percent of total revenue									34.00%	37.00%	35.00%
Operating Segments | Flat Rolled Products | Stainless steel sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue									20.00%	21.00%	19.00%
Operating Segments | Flat Rolled Products | Specialty stainless sheet
Disaggregation of Revenue [Line Items]
Percent of total revenue									10.00%	12.00%	11.00%
Operating Segments | Flat Rolled Products | Stainless steel plate and other
Disaggregation of Revenue [Line Items]
Percent of total revenue									4.00%	4.00%	5.00%
Operating Segments | Flat Rolled Products | United States
Disaggregation of Revenue [Line Items]
Revenue									$ 1,134.0	$ 974.1	$ 800.2
Operating Segments | Flat Rolled Products | China
Disaggregation of Revenue [Line Items]
Revenue									236.9	214.5	156.3
Operating Segments | Flat Rolled Products | Germany
Disaggregation of Revenue [Line Items]
Revenue									54.5	46.6	40.3
Operating Segments | Flat Rolled Products | United Kingdom
Disaggregation of Revenue [Line Items]
Revenue									9.7	10.7	12.7
Operating Segments | Flat Rolled Products | Japan
Disaggregation of Revenue [Line Items]
Revenue									78.4	36.5	38.5
Operating Segments | Flat Rolled Products | France
Disaggregation of Revenue [Line Items]
Revenue									10.9	7.8	7.7
Operating Segments | Flat Rolled Products | Rest of World
Disaggregation of Revenue [Line Items]
Revenue									188.0	167.5	148.5
Operating Segments | Flat Rolled Products | Aerospace & Defense
Disaggregation of Revenue [Line Items]
Revenue									194.2	149.2	151.2
Operating Segments | Flat Rolled Products | Oil & Gas
Disaggregation of Revenue [Line Items]
Revenue									471.3	354.3	234.3
Operating Segments | Flat Rolled Products | Automotive
Disaggregation of Revenue [Line Items]
Revenue									313.9	264.9	225.2
Operating Segments | Flat Rolled Products | Food Equipment & Appliances
Disaggregation of Revenue [Line Items]
Revenue									244.5	224.9	170.4
Operating Segments | Flat Rolled Products | Electrical Energy
Disaggregation of Revenue [Line Items]
Revenue									103.1	79.1	103.5
Operating Segments | Flat Rolled Products | Construction/Mining
Disaggregation of Revenue [Line Items]
Revenue									153.2	141.8	123.9
Operating Segments | Flat Rolled Products | Medical
Disaggregation of Revenue [Line Items]
Revenue									14.6	12.6	10.5
Operating Segments | Flat Rolled Products | Electronics/Computers/Communications
Disaggregation of Revenue [Line Items]
Revenue									149.0	147.2	106.3
Operating Segments | Flat Rolled Products | Other
Disaggregation of Revenue [Line Items]
Revenue									68.6	83.7	78.9
External Customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue									4,046.6	3,525.1	3,134.6
External Customers | Operating Segments | High Performance Materials & Components
Disaggregation of Revenue [Line Items]
Revenue									2,334.2	2,067.4	1,930.4
External Customers | Operating Segments | Flat Rolled Products
Disaggregation of Revenue [Line Items]
Revenue									$ 1,712.4	$ 1,457.7	$ 1,204.2