| Cover | Nov. 19, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 19, 2021 |
| Entity Registrant Name | INVESTORS BANCORP, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-36441 |
| Entity Tax Identification Number | 46-4702118 |
| Entity Address, Address Line One | 101 JFK Parkway, |
| Entity Address, City or Town | Short Hills, |
| Entity Address, State or Province | NJ |
| Entity Address, Postal Zip Code | 07078 |
| City Area Code | 973 |
| Local Phone Number | 924-5100 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common |
| Trading Symbol | ISBC |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001594012 |
| Amendment Flag | false |
| Document Information [Line Items] | |
| Document Period End Date | Nov. 19, 2021 |
