Cover - USD ($) $ in Millions	12 Months Ended
Cover - USD ($) $ in Millions	Dec. 31, 2025	Apr. 27, 2026	Jun. 30, 2025
Document Information [Line Items]
Document Type	10-K/A
Document Annual Report	true
Current Fiscal Year End Date	--12-31
Document Period End Date	Dec. 31, 2025
Document Transition Report	false
Entity File Number	001-38456
Entity Registrant Name	COLUMBIA FINANCIAL, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	22-3504946
Entity Address, Address Line One	19-01 Route 208 North
Entity Address, City or Town	Fair Lawn
Entity Address, State or Province	NJ
Entity Address, Postal Zip Code	07410
City Area Code	800
Local Phone Number	522-4167
Title of 12(b) Security	Common stock, par value $0.01 per share
Trading Symbol	CLBK
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Emerging Growth Company	false
Entity Filer Category	Accelerated Filer
Entity Small Business	false
ICFR Auditor Attestation Flag	true
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 354.6
Entity Common Stock, Shares Outstanding		104,142,951
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	FY
Entity Central Index Key	0001723596
Amendment Flag	true
Amendment Description	On March 3, 2026, Columbia Financial, Inc. (the “Company”), a Delaware corporation, filed its Annual Report on Form 10-K for the fiscal year ended December 31, 2025 (the “Original Form 10-K”). The Original Form 10-K omitted Part III, Item 10 (Directors, Executive Officers and Corporate Governance), Item 11 (Executive Compensation), Item 12 (Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters), Item 13 (Certain Relationships and Related Transactions, and Director Independence) and Item 14 (Principal Accountant Fees and Services) in reliance on General Instruction G(3) to Form 10-K, which provides that such information may be either incorporated by reference from the registrant’s definitive proxy statement or included in an amendment to Form 10-K, in either case filed with the Securities and Exchange Commission (the “SEC”) not later than 120 days after the end of the fiscal year. This Amendment No. 1 to Form 10-K (this “Amendment”) is being filed solely to: • Amend Part III, Items 10, 11, 12, 13 and 14 of the Original Form 10-K to include the information required by such Items; and • Include new certifications of our principal executive officer and principal financial officer as exhibits to this Amendment under Item 15 of Part IV hereof, pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”). The cover page of the Original Form 10-K is also amended to update the number of shares outstanding of the registrant’s common stock, which now reflects the amount outstanding as of April 27, 2026. This Amendment does not otherwise change or update any of the disclosures set forth in the Original Form 10-K and does not otherwise reflect any events occurring after the filing of the Original Form 10-K. Capitalized terms used but not otherwise defined herein shall have the respective meanings ascribed to such terms in the Original Form 10-K. As used in this Amendment, references the “Company” and “Columbia Financial” refer to Columbia Financial, Inc., a Delaware corporation, and the terms “we,” “us” or “our” refer to the Company and its subsidiary, Columbia Bank, collectively. Other terms used but not defined herein are as defined in the Original Form 10-K.

Columbia Financial, Inc. 10-K/A