Debt - Narrative (Details)			3 Months Ended			12 Months Ended
		Jun. 02, 2016 USD ($) $ / shares	Sep. 30, 2018	Dec. 31, 2017 USD ($)	Jun. 30, 2017	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jul. 31, 2018 USD ($)	May 24, 2016 USD ($)	Feb. 15, 2016	[1]	Dec. 31, 2013
Interest Costs Incurred [Abstract]
Interest expense						$ 102,100,000	$ 134,900,000	$ 125,400,000
Interest costs capitalized						4,100,000	2,600,000	4,700,000
Interest costs paid						102,600,000	133,800,000	127,200,000
Interest income						1,100,000	1,100,000	1,400,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2019						6,600,000
2020						4,700,000
2021						502,000,000
2022						388,800,000
2023						500,400,000.0
Debt Instrument [Line Items]
Payments of debt issuance costs						0	800,000	10,400,000
Debt extinguishment charge				$ 37,000,000		0	37,000,000	0
Payment for Debt Extinguishment or Debt Prepayment Cost						$ 0	35,800,000	0
Write off of Deferred Debt Issuance Cost							1,200,000
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%
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%
Asset-based lending credit facility [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity						$ 500,000,000
Domestic Bank Group $400 million credit agreement [Member]
Debt Instrument [Line Items]
Average outstanding amount						$ 43,000,000	$ 37,000,000
Interest rate during period						3.70%	3.30%
Domestic Bank Group $400 million asset-based credit facility [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount				0		$ 0	$ 0
Line of credit facility amount outstanding						35,300,000
2022 Term Loan [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage									5.44%
Payments of debt issuance costs							800,000	$ 1,000,000
Debt instrument carrying amount				$ 100,000,000		100,000,000	$ 100,000,000
Debt Instrument, Prepayment Increments, Minimum						$ 50,000,000
2022 Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Variable rate spread			2.50%		3.00%			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%	4.75%
Debt instrument carrying amount				$ 287,500,000		$ 287,500,000	$ 287,500,000
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%
Payments of debt issuance costs		$ 9,400,000
Amortization period of debt issuance costs		6 years
Long-term debt										$ 287,500,000
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 $350 million 9.375% Senior Notes due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage								9.375%
Debt instrument carrying amount								$ 350,000,000
STAL Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity						$ 20,000,000
Joint venture ownership percentage						60.00%
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							$ 1,000,000	$ 2,400,000
Fixed charge coverage ratio						1
Remaining borrowing capacity, percent of minimum borrowing capacity						10.00%
Minimum remaining borrowing capacity						$ 40,000,000
Minimum required liquidity						$ 700,000,000
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						1.75%		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						2.25%		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
Interest Rate Swap [Member] | 2022 Term Loan [Member]
Debt Instrument [Line Items]
Derivative notional amount									$ 50,000,000

[1] Bearing interest at 7.875% effective February 15, 2016.