Debt - Narrative (Details) $ / shares in Units, shares in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) tradingDays $ / shares shares	Dec. 31, 2021 USD ($) $ / shares	Dec. 31, 2020 USD ($)	Sep. 09, 2027 USD ($)	Sep. 09, 2022 USD ($)	Sep. 08, 2022 USD ($)	Jan. 01, 2021 USD ($)	Jul. 02, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2013
Interest Costs Incurred [Abstract]
Interest expense	$ 92,100,000	$ 97,600,000	$ 96,100,000
Interest costs capitalized	5,100,000	4,300,000	7,700,000
Interest costs paid	92,800,000	97,500,000	95,400,000
Interest income	4,700,000	700,000	1,700,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2023	41,700,000
2024	19,900,000
2025	457,800,000
2026	10,500,000
2027	557,400,000
Debt Instrument [Line Items]
Payments of debt issuance costs	0	9,500,000	9,100,000
Debt extinguishment charge	0	65,500,000	21,500,000
Payment for Debt Extinguishment or Debt Prepayment Cost	0	64,500,000	19,100,000
Debt Issuance Costs, Net	17,200,000	20,800,000
Purchase of convertible note capped call	0	0	(19,400,000)
Equity component of convertible note			49,800,000
Stockholders' equity, including portion attributable to noncontrolling interest	1,157,200,000	832,700,000	641,400,000						$ 2,193,200,000
Interest expense, net	87,400,000	96,900,000	94,400,000
Percentage of Forecast [Abstract]
Conversion of convertible notes	$ 82,500,000
Stock Issued During Period, Shares, Conversion of Convertible Securities | shares	5.7
Repayment of Long-term Debt, Long-term Lease Obligation, and Capital Security	$ 23,100,000	515,600,000	212,100,000
Pico Rivera, CA Operations
Percentage of Forecast [Abstract]
Proceeds from Sale of Other Assets	6,200,000
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value
Debt Instrument [Line Items]
Debt	1,964,500,000	2,003,200,000
Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Debt	1,665,700,000	1,690,100,000
Additional Paid-In Capital
Debt Instrument [Line Items]
Equity component of convertible note			49,800,000
Stockholders' equity, including portion attributable to noncontrolling interest	1,668,100,000	1,596,700,000	$ 1,625,500,000						$ 1,618,000,000
Percentage of Forecast [Abstract]
Conversion of convertible notes	45,400,000
Additional Paid-In Capital | Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest							$ (49,800,000)
ATI $500 million 5.875% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage										5.875%
Increase in interest expense per downgrade notch, percent			0.25%
Maximum number of downgrade notches			4
Maximum increase in interest rate expense, percent			2.00%
Debt instrument carrying amount			$ 500,000,000
Write off of Deferred Debt Issuance Cost		$ 1,000,000
ATI $500 million 5.875% Senior Notes due 2023 [Member] | Maximum
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage			7.875%
ATI Convertible Senior Notes, 4.75%, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage		4.75%
Debt instrument carrying amount	$ 0	$ 84,200,000
ATI Convertible Senior Notes, 4.75%, Due 2022 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage		4.75%
Convertible debt conversion rate in shares of stock		0.0692042
Conversion price of convertible notes | $ / shares		$ 14.45
Write off of Deferred Debt Issuance Cost			$ 2,400,000
Debt Instrument, Repurchased Face Amount			$ 203,200,000
Debt Issuance Costs, Net		$ 300,000
Debt Instrument, Interest Rate, Effective Percentage	5.40%	5.40%	5.40%
Interest Expense, Debt, Excluding Amortization	$ 2,000,000.0	$ 4,000,000.0	$ 8,600,000
Amortization of Debt Issuance Costs	300,000	500,000	1,000,000.0
Interest expense, net	$ 2,300,000	4,500,000	9,600,000
ATI Convertible Senior Notes, 4.75%, Due 2022 [Member] | Convertible Debt [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measurements at Reporting Date Using Total Estimated Fair Value
Debt Instrument [Line Items]
Debt		$ 102,000,000
ATI $350 million 5.875% Senior Notes due 2027
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	5.875%	5.875%
Debt instrument carrying amount	$ 350,000,000.0	$ 350,000,000.0
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	3.50%	3.50%
Debt instrument carrying amount	$ 291,400,000	$ 291,400,000
Convertible debt conversion rate in shares of stock	0.0645745
Number of equity instruments issuable	18,800,000
Conversion price of convertible notes | $ / shares	$ 15.49
Debt Instrument, Convertible, Redemption Price As A Percent Of Principal Amount	100.00%
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger	130.00%
Debt Instrument, Convertible, Threshold Trading Days | tradingDays	20
Debt Instrument, Convertible, Threshold Consecutive Trading Days | tradingDays	30
Equity component of convertible debt			$ 46,800,000					$ 51,400,000
Debt Issuance Costs, Net	$ 4,800,000	$ 6,500,000
Debt Instrument, Interest Rate, Effective Percentage	4.20%	4.20%	8.40%
Debt Instrument, Convertible, Cap Price | $ / shares	$ 19.76
Interest Expense, Debt, Excluding Amortization	$ 10,200,000	$ 10,200,000	$ 5,300,000
Amortization of Debt Issuance Costs	1,800,000	1,700,000	700,000
Interest expense, net	12,000,000.0	11,900,000	6,000,000.0
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measurements at Reporting Date Using Total Estimated Fair Value
Debt Instrument [Line Items]
Debt	$ 590,000,000	379,000,000
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member] | Additional Paid in Capital
Debt Instrument [Line Items]
Debt Issuance Costs, Net			$ 1,600,000
ATI 2029 and 2031 Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage	101.00%
Proceeds from Debt, Net of Issuance Costs		$ 665,700,000
ATI 2029 and 2031 Notes | Minimum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption	15 days
ATI 2029 and 2031 Notes | Maximum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption	60 days
ATI 2029 Notes
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	4.875%	4.875%
Payments of debt issuance costs		$ 4,700,000
Debt instrument carrying amount	$ 325,000,000.0	$ 325,000,000.0
Debt instrument, term	8 years
ATI 2031Notes
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	5.125%	5.125%
Payments of debt issuance costs		$ 4,700,000
Debt instrument carrying amount	$ 350,000,000.0	350,000,000.0
Debt instrument, term	10 years
ATI Inc., Convertible Senior Notes, 4.75%, Due 2022
Percentage of Forecast [Abstract]
Repayment of Long-term Debt, Long-term Lease Obligation, and Capital Security	$ 1,700,000
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Line of credit facility amount outstanding	39,800,000
Average outstanding amount	0	0
Percentage of Forecast [Abstract]
Bridge Loan	60,000,000
Short-Term Debt	$ 0	0
2027 Term Loan
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	4.21%
Debt instrument carrying amount	$ 200,000,000	$ 200,000,000.0
Debt Instrument, Prepayment Increments, Minimum	$ 25,000,000
2027 Term Loan | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate spread	2.00%
ATI Inc., Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt [Member] | Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest							$ (45,400,000)
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	$ 600,000,000
Fixed charge coverage ratio	1.00
Remaining borrowing capacity, percent of minimum borrowing capacity	10.00%
Minimum remaining borrowing capacity	$ 60,000,000
Debt Issuance Costs, Net					$ 2,400,000	$ 1,700,000
Debt Covenant, Undrawn Availability Requirement, Amount	$ 120,000,000
Debt Covenant, Undrawn Availability Requirement, Percent Of Total Available Liquidity	20.00%
Percentage of Forecast [Abstract]
Line of Credit Facility, Increase (Decrease), Net	$ 100,000,000
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility | Minimum | Base Rate [Member]
Debt Instrument [Line Items]
Variable rate spread	0.25%
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility | Minimum | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate spread	1.25%
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility | Maximum | Base Rate [Member]
Debt Instrument [Line Items]
Variable rate spread	0.75%
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility | Maximum | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate spread	1.75%
Letter of Credit [Member] | Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	$ 200,000,000
Foreign Line of Credit
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	60,000,000
Interest Rate Swap [Member] | 2027 Term Loan
Debt Instrument [Line Items]
Derivative notional amount	$ 50,000,000
Forecast | Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Line of Credit Facility, Increase to Maximum Borrowing Capacity				$ 300,000,000