Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization to net income (loss) of net prior service credits		$ (0.2)	$ (0.1)	$ (0.3)	$ (0.3)
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses		15.9	19.2	31.8	38.5
Settlement loss included in net income (loss)		0.0	29.5	0.0	29.5
Income (loss) before income taxes		82.8	(30.9)	159.3	9.2
Income tax provision		3.7	3.4	8.0	8.3
Reclassification adjustment included in net income (loss)		0.0	20.0	0.0	20.0
Sales		1,046.0	959.5	2,084.1	1,793.6
Cost of sales		836.9	784.2	1,681.8	1,448.9
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]	0.2	0.1	0.3	0.3
Amount reclassified from AOCI, Postretirement benefit plans, Actuarial losses	[1]	(15.9)	(19.2)	(31.8)	(38.5)
Settlement loss included in net income (loss)	[2]	0.0	(29.5)	0.0	(29.5)
Income (loss) before income taxes	[3]	(15.7)	(48.6)	(31.5)	(67.7)
Income tax provision	[4]	(3.7)	1.6	(7.4)	(3.0)
Net income (loss)		(12.0)	(50.2)	(24.1)	(64.7)
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 (loss)	[2],[3]	0.0	(20.0)	0.0	(20.0)
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	[3]	(3.8)	20.4	1.7	25.4
Income tax provision	[4]	(0.9)	4.9	0.4	6.1
Net income (loss)		(2.9)	15.5	1.3	19.3
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]	(1.8)	15.9	5.1	19.1
Cost of sales	[5]	(1.8)	15.9	5.1	19.1
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]	(2.4)	4.5	(4.2)	6.6
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]	0.0	0.1	0.1	0.1
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]	$ 0.4	$ (0.1)	$ 0.7	$ (0.4)

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

[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.

[3] For pre-tax 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 8).