Debt - Narrative (Details)						12 Months Ended
		Sep. 23, 2015 USD ($)	Jun. 02, 2014 USD ($)	Jun. 01, 2014 USD ($) shares	Jul. 12, 2013 USD ($)	Dec. 31, 2015 USD ($)		Dec. 31, 2014 USD ($)		Dec. 31, 2013 USD ($) $ / shares	Aug. 15, 2015	[1]
Interest Costs Incurred [Abstract]
Interest expense						$ 111,400,000		$ 109,800,000		$ 66,000,000
Interest costs capitalized						2,200,000		5,300,000		45,700,000
Interest costs paid						113,400,000		113,200,000		110,600,000
Interest income						1,200,000		1,100,000		800,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2016						3,900,000
2017						900,000
2018						200,000
2019						350,200,000
2020						0
Debt Instrument [Line Items]
Payments of debt issuance costs						0		1,200,000		$ 5,200,000
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount					$ 500,000,000	$ 500,000,000	[1]	$ 500,000,000	[1]
Debt instrument interest rate stated percentage								6.125%		5.875%	7.625%
Debt instrument maturity date					Aug. 15, 2023
Payments of debt issuance costs										$ 5,200,000
Amortization period of debt issuance costs						10 years
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount						$ 350,000,000		$ 350,000,000
Debt instrument interest rate stated percentage						9.375%		9.375%
Allegheny Technologies $402.5 million 4.25% Convertible Senior Notes due 2014 [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount										$ 402,500,000
Debt instrument interest rate stated percentage										4.25%
Repayments of Convertible Debt			$ 397,500,000
Convertible notes, Amount converted				$ 5,000,000
Convertible notes, Shares issued | shares				120,476
Convertible debt conversion rate in shares of stock				0.0239263
Conversion price of convertible notes | $ / shares										$ 41.795
Ladish Series B 6.14% Notes due 2016 [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount	[2]					$ 0		$ 11,900,000
Debt instrument interest rate stated percentage	[2]					6.14%		6.14%
Repayments of Debt						$ 5,700,000
Ladish Series C 6.41% Notes due 2015 [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount	[3]					$ 0		$ 10,300,000
Debt instrument interest rate stated percentage	[3]					6.41%		6.41%
Repayments of Debt						$ 10,000,000
Domestic Bank Group $400 million unsecured credit agreement [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount								$ 0
Seperate Letter of Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility amount outstanding						$ 32,000,000
STAL Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Joint venture ownership percentage						60.00%
Maximum [Member] | Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage						7.875%
Domestic Bank Group $400 million credit agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Average Outstanding Amount						$ 61,000,000
Line of Credit Facility, Interest Rate During Period						2.00%
STAL Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity						$ 20,000,000
Allegheny Technologies, Senior Notes, 5.875, Due 2023 [Member]
Debt Instrument [Line Items]
Increase in stated interest rate due to downgrade (percent)						1.75%
Additional Annual Interest Expense to be Incurred						$ 4,100,000
Credit Downgrade, Increase in Interest Expense per Downgrade Notch, Percent						0.25%
Credit Downgrade, Maximum Number of Downgrade Notches						4
Credit Downgrade, Maximum Increase in Interest Rate Expense, Percent						2.00%
Domestic Bank Group $400 million asset-based credit facility [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount						$ 0
Line of credit facility amount outstanding						$ 4,600,000
Revolving Credit Facility [Member] | Domestic Bank Group $400 million credit agreement [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity								$ 400,000,000
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member]
Debt Instrument [Line Items]
Amortization period of debt issuance costs		5 years
Line of credit maximum borrowing capacity		$ 400,000,000
Debt Issuance Cost		1,500,000
Debt Instrument, Covenant, Remaining Borrowing Capacity		40,000,000
Debt Instrument, Covenant, Minimum Required Liquidity		$ 500,000,000
Debt Instrument, Covenant, Minimum Liquidity, Number of Days Prior		91 days
Debt Instrument, Covenant, Fixed Charge Coverage Ratio		1
Debt Instrument, Covenant, Remaining Borrowing Capacity, Percent Minimum Borrowing Capacity		10.00%
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable rate spread		1.25%
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Minimum [Member] | Base Rate [Member]
Debt Instrument [Line Items]
Variable rate spread		0.25%
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable rate spread		1.75%
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Maximum [Member] | Base Rate [Member]
Debt Instrument [Line Items]
Variable rate spread		0.75%
Letter of Credit [Member] | Domestic Bank Group $400 million asset-based credit facility [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity		$ 200,000,000

[1] Bearing interest at 7.625% effective August 15, 2015.

[2] Includes fair value adjustment of $0.4 million at December 31, 2014.

[3] Includes fair value adjustment of $0.3 million at December 31, 2014.