Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022		Sep. 30, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income of net prior service credits		$ (0.1)	$ (0.4)	$ (0.4)		$ (1.4)
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses		19.1	22.5	57.6		66.9
Settlement loss (gain) included in net income		0.0	(21.9)	29.5		(21.9)
Income before income taxes		67.4	77.2	76.6		39.9
Income tax provision		3.0	22.0	11.3		31.5
Sales		1,032.0	725.7	2,825.6		2,034.4
Cost of sales		848.2	643.2	2,297.1		1,823.4
Reclassification adjustment included in net income		0.0	0.0	20.0		0.0
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 of net prior service credits	[1]	0.1	0.4	0.4		1.4
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses	[1]	(19.1)	(22.5)	(57.6)		(66.9)
Settlement loss (gain) included in net income	[2]	0.0	21.9	(29.5)		21.9
Income before income taxes	[3]	(19.0)	(0.2)	(86.7)		(43.6)
Income tax provision	[4]	(4.5)	3.8	(7.5)		(6.6)
Net income (loss)		(14.5)	(4.0)	(79.2)		(37.0)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	[3]	7.1	3.0	32.5		6.2
Income tax provision	[4]	1.7	0.7	7.8		1.5
Net income (loss)		5.4	2.3	24.7		4.7
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]
Sales	[5]	1.0	2.0	20.1		5.4
Cost of sales	[5]	1.0	2.0	20.1		5.4
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	[5]	5.4	1.3	12.0		1.6
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivatives [Member] | Foreign Exchange
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of sales	[5]	0.7	0.0	0.8		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	[5]	0.0	(0.3)	(0.4)		(0.8)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Currency translation adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment included in net income	[3]	$ 0.0	$ 0.0	$ (20.0)	[2]	$ 0.0

[1] Amounts are reported in nonoperating retirement benefit expense (see Note 12).

[2] Amounts in 2022 were included in loss on asset sales and sales of businesses, net, as part of the loss on sale of the Sheffield, UK operations (see Note 5). Amounts in 2021 are reported in nonoperating retirement benefit expense (see Note 12).

[3] 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.

[4] These amounts exclude the impact of any deferred tax asset valuation allowances, when applicable.

[5] Amounts related to derivatives, with the exception of the interest rate swap, are included in sales or 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 9).