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Business Segments - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
Business Segments - Narrative (Details) $ in Millions	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment		2
Restructuring charges		$ 0.0	$ 0.0	$ 523.8
Income (loss) from equity method investments		(1.0)	0.0	0.0
Total international sales		1,698.4	1,454.5	1,277.1
Sales by U.S. operations to foreign countries		1,303.8	1,078.6	971.4
LIFO-related net realizable value charge		(2.9)	(43.1)
Inventory valuation reserves		88.5	121.5
Closed operations and other expenses		$ 21.6	34.0	34.6
STAL Precision Stainless Steel Company Limited [Member]
Segment Reporting Information [Line Items]
Joint venture ownership percentage		60.00%
Allegheny & Tsingshan Stainless [Member]
Segment Reporting Information [Line Items]
Equity method investment ownership percentage		50.00%
Revenue from related parties		$ 4.1
Allegheny & Tsingshan Stainless [Member] \| Flat Rolled Products
Segment Reporting Information [Line Items]
Income (loss) from equity method investments		$ (3.9)
Uniti [Member]
Segment Reporting Information [Line Items]
Equity method investment ownership percentage		50.00%
Revenue from related parties		$ 49.4	38.6	20.3
Uniti [Member] \| Flat Rolled Products
Segment Reporting Information [Line Items]
Income (loss) from equity method investments		$ 2.9	$ 0.6	0.5
Uniti [Member] \| VSMPO [Member]
Segment Reporting Information [Line Items]
Joint venture ownership percentage by unaffiliated entity		50.00%
SEC Schedule, 12-09, Reserve, Inventory \| Titanium sponge inventory
Segment Reporting Information [Line Items]
Inventory valuation charges	$ 11.3
Employee benefit costs [Member]
Segment Reporting Information [Line Items]
Restructuring charges				20.8
Employee benefit costs [Member] \| Flat Rolled Products
Segment Reporting Information [Line Items]
Restructuring charges				4.9
Employee benefit costs [Member] \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Restructuring charges				7.5
Long-lived asset impairment [Member]
Segment Reporting Information [Line Items]
Restructuring charges				471.3
Facility Closing [Member]
Segment Reporting Information [Line Items]
Restructuring charges				31.7
Facility Closing [Member] \| Flat Rolled Products
Segment Reporting Information [Line Items]
Restructuring charges				8.4
Facility Closing [Member] \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Restructuring charges				23.8
Employee benefit costs including special termination benefits [Member]
Segment Reporting Information [Line Items]
Restructuring charges				$ 24.2
Aerospace & Defense Market Concentration Risk [Member] \| Sales Revenue, Net [Member] \| High Performance Materials & Components
Segment Reporting Information [Line Items]
Percentage of sales		75.00%
Oil & Gas and Automotive Market Concentration [Member] \| Sales Revenue, Net [Member] \| Flat Rolled Products
Segment Reporting Information [Line Items]
Percentage of sales		45.00%
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member]
Segment Reporting Information [Line Items]
Debt Instrument, Interest Rate, Stated Percentage				9.375%