| Cover Page - USD ($) $ in Millions | 12 Months Ended | ||
| Dec. 31, 2021 | Mar. 22, 2022 | Jun. 30, 2021 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Document Annual Report | true | ||
| Document Period End Date | Dec. 31, 2021 | ||
| Current Fiscal Year End Date | --12-31 | ||
| Document Transition Report | false | ||
| Entity File Number | 001-36441 | ||
| Entity Registrant Name | Investors Bancorp Inc. | ||
| Entity Incorporation, State | DE | ||
| Entity Tax Identification Number | 46-4702118 | ||
| Entity Address, Street | 101 JFK Parkway, | ||
| Entity Address, City | Short Hills, | ||
| Entity Address, State | NJ | ||
| Entity Address, Postal Zip Code | 07078 | ||
| City Area Code | 973 | ||
| Local Phone Number | 924-5100 | ||
| Title of each class | Common stock | ||
| Trading Symbol(s) | ISBC | ||
| Name of each exchange on which registered | NASDAQ | ||
| Entity Well-known Seasoned Issuer | Yes | ||
| Entity Voluntary Filers | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Interactive Data Current | Yes | ||
| Entity Filer Category | Large Accelerated Filer | ||
| Entity Small Business | false | ||
| Entity Emerging Growth Company | false | ||
| ICFR Auditor Attestation Flag | true | ||
| Entity Shell Company | false | ||
| Entity Common Stock, Shares Outstanding | 249,158,823 | ||
| Entity Public Float | $ 3,230 | ||
| Documents Incorporated by Reference | None. | ||
| Entity Central Index Key | 0001594012 | ||
| Document Fiscal Year Focus | 2021 | ||
| Document Fiscal Period Focus | FY | ||
| Amendment Flag | true | ||
| Amendment Description | This Amendment No. 1 to the Annual Report on Form 10-K (this “Amendment”) amends the Annual Report on Form 10-K for the fiscal year ended December 31, 2021 originally filed on March 1, 2022 (the “Original Filing”) by Investors Bancorp, Inc., a Delaware corporation (“Investors” or the “Company”). Investors is filing this Amendment to present the information required by Part III of Form 10-K as the Company will not file its definitive proxy statement within 120 days of the end of its fiscal year ended December 31, 2021. Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, as amended, this Amendment also contains new certifications by the principal executive officer and the principal financial officer as required by Section 302 of the Sarbanes-Oxley Act of 2002. Accordingly, Item 15(a)(3) of Part IV is amended to include the currently dated certifications as exhibits. Because no financial statements have been included in this Amendment and this Amendment does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4 and 5 of the certifications have been omitted. On July 28, 2021, the Company and Citizens Financial Group, Inc. (“Citizens”) entered into an Agreement and Plan of Merger dated as of July 28, 2021 (the “Merger Agreement”) pursuant to which Citizens will acquire the Company. The transaction was approved by the Company’s stockholders on November 19, 2021. On March 22, Citizens and Investors received regulatory approval from the Board of Governors of the Federal Reserve System. On March 24, 2022, Citizens and Investors received regulatory approval from the Office of the Comptroller of the Currency. The transaction is expected to occur as of the close of business on April 6, 2022, therefore the Company does not anticipate holding an annual meeting of stockholders prior to closing. Except as described above or as expressly noted in this Amendment, no other changes have been made to the Original Filing. The Original Filing continues to speak as of the date of the Original Filing, and the Company has not updated the disclosures contained therein to reflect any events which occurred at a date subsequent to the filing of the Original Filing. | ||
| Auditor Name | KPMG LLP, | ||
| Auditor Firm ID | 185 | ||
| Auditor Location | Short Hills, NJ |
