Document and Entity Information - USD ($)	6 Months Ended
	Jun. 30, 2017	Jul. 21, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Document Period End Date	Jun. 30, 2017
Amendment Flag	false
Entity Registrant Name	ALLEGHENY TECHNOLOGIES INCORPORATED
Entity Central Index Key	0001018963
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Common Stock, Shares Outstanding		108,879,684
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q2
Entity Public Float			$ 1,400,000,000