Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 29, 2024	Dec. 31, 2023
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Derivative Asset Statement Of Financial Position Extensible Enumeration Not Disclosed Flag	Assets
Derivative Liability Statement Of Financial Position Extensible Enumeration Not Disclosed Flag	Liabilities
Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 721.2	$ 743.9
Derivative financial instruments: Assets	0.0	0.0
Derivative financial instruments: Liabilities	0.0	0.0
Debt	1,580.2	2,438.9
Fair Value Measurements at Reporting Date Using Significant Observable Inputs (Level 2)
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Derivative financial instruments: Assets	1.9	0.9
Derivative financial instruments: Liabilities	6.0	14.2
Debt	309.5	307.8
Total Carrying Amount
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	721.2	743.9
Derivative financial instruments: Assets	1.9	0.9
Derivative financial instruments: Liabilities	6.0	14.2
Debt	1,909.5	2,199.2
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	721.2	743.9
Derivative financial instruments: Assets	1.9	0.9
Derivative financial instruments: Liabilities	6.0	14.2
Debt	$ 1,889.7	2,746.7
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) | ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt		$ 864.0