Debt - Narrative (Details)						1 Months Ended	3 Months Ended	12 Months Ended
		Jun. 02, 2016 USD ($) $ / shares	Sep. 23, 2015 USD ($)	Jun. 02, 2014 USD ($)	Jun. 01, 2014 USD ($) shares	Jul. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	May 24, 2016 USD ($)	Feb. 15, 2016	[1]	Dec. 31, 2013 $ / shares
Interest Costs Incurred [Abstract]
Interest expense								$ 125,400,000	$ 111,400,000	$ 109,800,000
Interest costs capitalized								4,700,000	2,200,000	5,300,000
Interest costs paid								127,200,000	113,400,000	113,200,000
Interest income								1,400,000	1,200,000	1,100,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2017							$ 105,800,000	105,800,000
2018							700,000	700,000
2019							350,200,000	350,200,000
2020							0	0
2021							500,000,000	500,000,000
Debt Instrument [Line Items]
Payments of debt issuance costs								$ 10,400,000	0	$ 1,200,000
Employer contributions						$ 115,000,000
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage												7.875%		5.875%
Increase in interest expense per downgrade notch, percent								0.25%
Maximum number of downgrade notches								4
Maximum increase in interest rate expense, percent								2.00%
Additional annual interest expense to be incurred								$ 1,300,000	4,100,000
Debt instrument carrying amount	[1]						$ 500,000,000	$ 500,000,000	500,000,000
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage							7.875%	7.875%
Domestic Bank Group $400 million credit agreement [Member]
Debt Instrument [Line Items]
Average outstanding amount								$ 82,000,000	$ 61,000,000
Interest rate during period								1.757%	2.00%
Domestic Bank Group $400 million asset-based credit facility [Member]
Debt Instrument [Line Items]
Payments of debt issuance costs								$ 900,000
Debt instrument carrying amount							$ 0	0	$ 0
Line of credit facility amount outstanding							25,900,000	25,900,000
2017 Term Loan [Member]
Debt Instrument [Line Items]
Payments of debt issuance costs								1,000,000
Debt instrument carrying amount							$ 100,000,000	$ 100,000,000	0
2017 Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable rate spread							3.50%
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage							4.75%	4.75%
Debt instrument carrying amount							$ 287,500,000	$ 287,500,000	$ 0
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage							4.75%	4.75%
Payments of debt issuance costs		$ 9,400,000
Amortization period of debt issuance costs							6 years
Long-term debt		$ 37,500,000									$ 250,000,000
Convertible debt conversion rate in shares of stock		0.0692042
Number of equity instruments issuable		19,900,000
Conversion price of convertible notes | $ / shares		$ 14.45
Allegheny Technologies $402.5 million 4.25% Convertible Senior Notes due 2014 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage										4.25%
Debt instrument carrying amount										$ 402,500,000
Convertible debt conversion rate in shares of stock					0.0239263
Conversion price of convertible notes | $ / shares														$ 41.795
Repayments of convertible debt				$ 397,500,000
Amount of debt converted					$ 5,000,000
Shares issued for converted debt | shares					120,476
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage							9.375%	9.375%	9.375%
Debt instrument carrying amount							$ 350,000,000	$ 350,000,000	$ 350,000,000
Ladish Series B 6.14% Notes due 2016 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage										6.14%
Repayments of debt									5,700,000
Ladish Series C 6.41% Notes due 2015 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage										6.41%
Repayments of debt									$ 10,000,000
Seperate Letter of Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility amount outstanding							17,000,000	17,000,000
STAL Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity							$ 20,000,000	$ 20,000,000
Joint venture ownership percentage							60.00%	60.00%
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]
Line of credit maximum borrowing capacity			$ 400,000,000
Payments of debt issuance costs			$ 1,500,000
Amortization period of debt issuance costs			5 years
Fixed charge coverage ratio							1	1
Remaining borrowing capacity, percent of minimum borrowing capacity							12.50%	12.50%
Line of Credit Facility, Remaining Borrowing Capacity							$ 62,500,000	$ 62,500,000
Minimum remaining borrowing capacity							40,000,000	40,000,000
Minimum required liquidity							$ 500,000,000	$ 500,000,000
Minimum required liquidity, number of days prior								91 days
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								2.00%
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								1.00%
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								2.50%
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								1.50%
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 7.875% effective February 15, 2016.