Debt (Tables)

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Schedule of debt

Debt at December 31, 2023 and January 1, 2023 was as follows:

	Fiscal Year
(In millions)	2023		2022
ATI Inc. 7.25% Notes due 2030	$	425.0	$	—
ATI Inc. 5.875% Senior Notes due 2027	350.0		350.0
ATI Inc. 5.125% Senior Notes due 2031	350.0		350.0
ATI Inc. 4.875% Notes due 2029	325.0		325.0
ATI Inc. 3.5% Convertible Senior Notes due 2025	291.4		291.4
Allegheny Ludlum 6.95% Debentures due 2025 (a)	150.0		150.0
ABL Term Loan	200.0		200.0
U.S. revolving credit facility	—		—
Foreign credit agreements	5.0		19.4
Finance leases and other	102.8		79.4
Debt issuance costs	(19.6)		(17.2)
Total short-term and long-term debt	2,179.6		1,748.0
Short-term debt and current portion of long-term debt	31.9		41.7
Total long-term debt	$	2,147.7	$	1,706.3

(a)The payment obligations of these debentures issued by Allegheny Ludlum, LLC are fully and unconditionally guaranteed by ATI.

Interest Income and Interest Expense Disclosure

Interest expense on the 2025 Convertible Notes was as follows:

	Fiscal Year
(in millions)	2023		2022		2021
Contractual coupon rate	$	10.2	$	10.2	$	10.2
Amortization of debt issuance costs	1.9		1.8		1.7
Total interest expense	$	12.1	$	12.0	$	11.9

Interest expense on the 2022 Convertible Notes was as follows:

	Fiscal Year
(in millions)	2022		2021
Contractual coupon rate	$	2.0	$	4.0
Amortization of debt issuance costs	0.3		0.5
Total interest expense	$	2.3	$	4.5