Cover - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Sep. 27, 2025	Nov. 24, 2025	Mar. 28, 2025
Document Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Financial Statement Error Correction [Flag]	false
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	true
Amendment Flag	false
Document Period End Date	Sep. 27, 2025
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	FY
Entity Information [Line Items]
Entity Registrant Name	Magnera Corporation
Entity Central Index Key	0000041719
Entity File Number	1-03560
Entity Tax Identification Number	23-0628360
Entity Incorporation, State or Country Code	PA
Current Fiscal Year End Date	--09-27
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Public Float			$ 650.8
Entity Contact Personnel [Line Items]
Entity Address, Address Line One	9335 Harris Corners Pkwy
Entity Address, Address Line Two	Suite 300
Entity Address, City or Town	Charlotte
Entity Address, State or Province	NC
Entity Address, Postal Zip Code	28269
Entity Phone Fax Numbers [Line Items]
City Area Code	866
Local Phone Number	744-7380
Entity Listings [Line Items]
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	MAGN
Security Exchange Name	NYSE
Entity Common Stock, Shares Outstanding		35.6