Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service credits		$ (0.8)	$ (0.7)	$ (2.3)	$ (1.9)
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses		21.3	21.9	63.9	65.4
Income (loss) before income taxes		(46.2)	119.0	(339.1)	220.4
Income tax provision		0.8	3.7	104.2	10.3
Cost of sales		(559.9)	(859.0)	(2,076.2)	(2,635.4)
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 credits	[1]	0.8	0.7	2.3	1.9
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses	[1]	(21.3)	(21.9)	(63.9)	(65.4)
Income (loss) before income taxes	[2]	(20.5)	(21.2)	(61.6)	(63.5)
Income tax provision	[3]	(4.9)	(5.1)	(14.7)	(15.0)
Net income (loss)		(15.6)	(16.1)	(46.9)	(48.5)
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]	(0.4)	0.8	(6.4)	(0.6)
Income tax provision	[3]	(0.1)	0.1	(1.5)	(0.2)
Net income (loss)		(0.3)	0.7	(4.9)	(0.4)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Nickel and other raw material contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	0.9	1.5	(1.7)	0.7
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Natural gas contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	(1.1)	(0.8)	(3.7)	(0.8)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Foreign exchange contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[4]	0.0	0.4	(0.1)	0.0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Interest Rate Swap
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest Expense	[4]	$ (0.2)	$ (0.3)	$ (0.9)	$ (0.5)

[1] Amounts are reported in nonoperating retirement benefit expense (see Note 11).

[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.

[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 8).