Basis of Financial Statement Presentation	3 Months Ended
Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Financial Statement Presentation	Basis of Financial Statement Presentation The accompanying unaudited consolidated financial statements include the accounts of Columbia Financial, Inc., its wholly-owned subsidiary Columbia Bank (the "Bank") and the Bank's wholly-owned subsidiaries (collectively, the “Company”). In consolidation, all intercompany accounts and transactions are eliminated. Columbia Financial, Inc. is a majority-owned subsidiary of Columbia Bank, MHC (the "MHC"). The accounts of the MHC are not consolidated in the accompanying consolidated financial statements of the Company. In preparing the interim unaudited consolidated financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the dates of the consolidated statements of financial condition and consolidated statements of income for the periods presented. Actual results could differ from these estimates. Material estimates that are particularly susceptible to change are the determination of the adequacy of the allowance for loan losses, evaluation of the need for valuation allowances on deferred tax assets, and determination of liabilities related to retirement and other post-retirement benefits. These estimates and assumptions are evaluated on an ongoing basis and are adjusted when facts and circumstances dictate. The interim unaudited consolidated financial statements reflect all normal and recurring adjustments, which are, in the opinion of management, considered necessary for a fair presentation of the financial condition and results of operations for the periods presented. The results of operation for the three months ended March 31, 2019 are not necessarily indicative of the results of operation that may be expected for all of 2019. Certain reclassifications have been made in the consolidated financial statements to conform with current year classifications. The interim unaudited consolidated financial statements of the Company presented herein have been prepared in accordance with the rules of the Securities and Exchange Commission (“SEC”) for quarterly reports on Form 10-Q and U.S. generally accepted accounting principles (“GAAP”). Certain information and note disclosures have been condensed or omitted pursuant to the rules and regulations of the SEC. These unaudited consolidated financial statements should be read in conjunction with the Company's Annual Report on Form 10-K for the year ended December 31, 2018 and the audited consolidated financial statements included therein.

Basis of Financial Statement Presentation

The accompanying unaudited consolidated financial statements include the accounts of Columbia Financial, Inc., its wholly-owned subsidiary Columbia Bank (the "Bank") and the Bank's wholly-owned subsidiaries (collectively, the “Company”). In consolidation, all intercompany accounts and transactions are eliminated.

Columbia Financial, Inc. is a majority-owned subsidiary of Columbia Bank, MHC (the "MHC"). The accounts of the MHC are not consolidated in the accompanying consolidated financial statements of the Company.

In preparing the interim unaudited consolidated financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the dates of the consolidated statements of financial condition and consolidated statements of income for the periods presented. Actual results could differ from these estimates. Material estimates that are particularly susceptible to change are the determination of the adequacy of the allowance for loan losses, evaluation of the need for valuation allowances on deferred tax assets, and determination of liabilities related to retirement and other post-retirement benefits. These estimates and assumptions are evaluated on an ongoing basis and are adjusted when facts and circumstances dictate.

The interim unaudited consolidated financial statements reflect all normal and recurring adjustments, which are, in the opinion of management, considered necessary for a fair presentation of the financial condition and results of operations for the periods presented. The results of operation for the three months ended March 31, 2019 are not necessarily indicative of the results of operation that may be expected for all of 2019. Certain reclassifications have been made in the consolidated financial statements to conform with current year classifications.

The interim unaudited consolidated financial statements of the Company presented herein have been prepared in accordance with the rules of the Securities and Exchange Commission (“SEC”) for quarterly reports on Form 10-Q and U.S. generally accepted accounting principles (“GAAP”). Certain information and note disclosures have been condensed or omitted pursuant to the rules and regulations of the SEC.

These unaudited consolidated financial statements should be read in conjunction with the Company's Annual Report on Form 10-K for the year ended December 31, 2018 and the audited consolidated financial statements included therein.