Accumulated Other Comprehensive Income (Loss) (Details2) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service cost (credits)		$ (3.1)	$ (2.6)	$ (2.6)
Amortization of net actuarial loss		85.3	87.2	76.5
Cost of sales		(2,689.3)	(3,484.7)	(3,416.3)
Income (loss) before income taxes		(1,481.9)	241.6	247.7
Income tax provision (benefit)		77.7	(28.5)	11.0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Post- retirement benefit plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service cost (credits)	[1]	3.1	2.6	2.6
Amortization of net actuarial loss	[1]	(85.3)	(87.2)	(76.5)
Income (loss) before income taxes	[2]	(82.2)	(84.6)	(73.9)
Income tax provision (benefit)	[3]	(19.7)	(20.3)	(18.0)
Income (loss)		(62.5)	(64.3)	(55.9)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) before income taxes	[2]	(6.0)	4.1	11.7
Income tax provision (benefit)	[3]	(1.4)	1.0	2.8
Income (loss)		(4.6)	3.1	8.9
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Nickel and other raw material contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	(0.8)	5.1	10.2
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Natural gas contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	(3.7)	(1.2)	0.5
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Foreign Exchange Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	(0.1)	0.7	1.3
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Interest Rate Swap [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest Expense	[4]	$ (1.4)	$ (0.5)	$ (0.3)

[1] Amounts are included in nonoperating retirement benefit expense (see Note 16).

[2] For pretax items, positive amounts are income and negative amounts are expense in terms of the impact to net income. Tax effects are presented in conformity with ATI’s presentation in the consolidated statements of operations.

[3] These amounts exclude the impact of any deferred tax asset valuation allowances, when applicable, including recognition of stranded balances (see Note 19 for further explanation).

[4] Amounts related to derivatives, with the exception of the interest rate swap are included in cost of goods sold in the period or periods the hedged item affects earnings. Amounts related to the interest rate swap are included in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings (see Note 14).