Business	12 Months Ended
Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Business	Business On April 19, 2018, Columbia Financial, Inc. completed its minority stock offering, after receiving all regulatory approvals. In connection with the closing, 62,580,155 shares of its common stock was issued to Columbia Bank, MHC (the "MHC"), the mutual holding company of Columbia Financial, Inc., 3,476,675 shares to the Columbia Bank Foundation, Columbia Bank's charitable foundation, and 49,832,345 shares to depositors who subscribed for and were allocated shares in the minority stock offering, as well as the Columbia Bank Employee Stock Ownership Plan (the "ESOP"). The accounts of the MHC are not consolidated in the consolidated financial statements of the Company. On May 22, 2018, the Board of Directors of the Company adopted a resolution to change the Company’s fiscal year end from September 30 to December 31, effective immediately as of the date of the Board resolution. In addition, on May 22, 2018, the Boards of Directors of the MHC and the Bank also adopted resolutions to change the MHC’s and the Bank’s fiscal year ends from September 30 to December 31, effective immediately as of the date of the Board resolutions.

Business

On April 19, 2018, Columbia Financial, Inc. completed its minority stock offering, after receiving all regulatory approvals. In connection with the closing, 62,580,155 shares of its common stock was issued to Columbia Bank, MHC (the "MHC"), the mutual holding company of Columbia Financial, Inc., 3,476,675 shares to the Columbia Bank Foundation, Columbia Bank's charitable foundation, and 49,832,345 shares to depositors who subscribed for and were allocated shares in the minority stock offering, as well as the Columbia Bank Employee Stock Ownership Plan (the "ESOP"). The accounts of the MHC are not consolidated in the consolidated financial statements of the Company.

On May 22, 2018, the Board of Directors of the Company adopted a resolution to change the Company’s fiscal year end from September 30 to December 31, effective immediately as of the date of the Board resolution. In addition, on May 22, 2018, the Boards of Directors of the MHC and the Bank also adopted resolutions to change the MHC’s and the Bank’s fiscal year ends from September 30 to December 31, effective immediately as of the date of the Board resolutions.