Debt Narrative (Details)				3 Months Ended	9 Months Ended
		Oct. 23, 2015 USD ($)	Sep. 23, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Feb. 15, 2015	Dec. 31, 2014 USD ($)	Dec. 31, 2013
STAL Precision Stainless Steel Company Limited
Debt Instrument [Line Items]
Ownership interest (percentage)				60.00%	60.00%
ATI Ladish Series B 6.14% Notes due 2016 (b)
Debt Instrument [Line Items]
Interest rate							6.14%
Repayment of debt during period				$ 5,700,000
ATI Ladish Series C 6.41% Notes due 2015 (c)
Debt Instrument [Line Items]
Interest rate							6.41%
Repayment of debt during period				$ 10,000,000
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Interest rate				6.625%	6.625%	6.625%	6.125%	5.875%
Outstanding borrowings	[1]			$ 500,000,000	$ 500,000,000		$ 500,000,000
Domestic Bank Group $400 million asset-based credit facility
Debt Instrument [Line Items]
Outstanding borrowings				0	0
Amount utilized to support the issuance of letters of credit				4,600,000	4,600,000
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 400,000,000
Debt issuance costs			$ 1,300,000
Term of facility (in years)			5 years
Minimum fixed charge coverage ratio allowed in event of default			1
Minimum remaining borrowing capacity as a percent of maximum borrowing capacity			10.00%
Minimum remaining borrowing capacity			$ 40,000,000
Minimum required liquidity prior to maturity of 9.375% Senior Notes due 2019			$ 500,000,000
Minimum required liquidity number of days prior to maturity of 9.375% Senior Notes due 2019			91 days
Outstanding borrowings				0	0
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility | Minimum | LIBOR based borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)			1.25%
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility | Minimum | Base rate borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)			0.25%
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility | Maximum | LIBOR based borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)			1.75%
Domestic Bank Group $400 million asset-based credit facility | Revolving credit facility | Maximum | Base rate borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)			0.75%
Domestic Bank Group $400 million asset-based credit facility | Letter of credit sub-facility
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 200,000,000
Domestic Bank Group $400 million credit facility
Debt Instrument [Line Items]
Outstanding borrowings							0
Average borrowings during period					$ 49,200,000
Interest rate during period					2.40%
Domestic Bank Group $400 million credit facility | Revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity							$ 400,000,000
Seperateletter of Credit Facility
Debt Instrument [Line Items]
Letters of credit outstanding				32,000,000	$ 32,000,000
STAL Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 20,000,000	$ 20,000,000
Subsequent Event | Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Interest rate		7.125%
Additional annual interest expense to be incurred		$ 2,500,000
Increase in interest expense per downgrade notch (percent)		0.25%
Maximum number of downgrade notches		4
Maximum potential increase in interest rate expense (percent)		2.00%
Increase in stated interest rate due to credit downgrade (percent)		1.25%

[1] Bearing interest at 6.625% effective February 15, 2015.