Basis of Financial Statement Presentation	6 Months Ended
Jun. 30, 2018
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 significant inter-company accounts and transactions are eliminated. Columbia Financial, Inc. is a majority-owned subsidiary of Columbia Bank, MHC ("MHC"). The accounts of MHC are not consolidated in the accompanying consolidated financial statements of the Company. The Company owns 100% of the common stock of a Delaware statutory basis trust, Columbia Capital Trust I (the "Trust"). The trust is classified as a variable interest entity and is not consolidated as it does not satisfy the conditions for consolidation. 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 date of the consolidated balance sheets and consolidated statements of income for the periods presented. Material estimates that are particularly susceptible to change are: the allowance for loan losses, the valuation of securities, the valuation of post-retirement benefits and the valuation of deferred tax assets. Estimates and assumptions are reviewed periodically and the effects of revisions are reflected in the consolidated financial statements in the period they are deemed necessary. While management uses its best judgment, actual amounts or results could differ significantly from those estimates. 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 in the United States of America (“GAAP”). Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to the rules and regulations of the SEC. In the opinion of management, all adjustments and disclosures considered necessary for the fair presentation of the accompanying unaudited consolidated financial statements have been included. Interim results are not necessarily reflective of the results of the entire year.

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 significant inter-company accounts and transactions are eliminated.

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

The Company owns 100% of the common stock of a Delaware statutory basis trust, Columbia Capital Trust I (the "Trust"). The trust is classified as a variable interest entity and is not consolidated as it does not satisfy the conditions for consolidation.

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 date of the consolidated balance sheets and consolidated statements of income for the periods presented. Material estimates that are particularly susceptible to change are: the allowance for loan losses, the valuation of securities, the valuation of post-retirement benefits and the valuation of deferred tax assets. Estimates and assumptions are reviewed periodically and the effects of revisions are reflected in the consolidated financial statements in the period they are deemed necessary. While management uses its best judgment, actual amounts or results could differ significantly from those estimates.

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 in the United States of America (“GAAP”). Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to the rules and regulations of the SEC.

In the opinion of management, all adjustments and disclosures considered necessary for the fair presentation of the accompanying unaudited consolidated financial statements have been included. Interim results are not necessarily reflective of the results of the entire year.