| Cover Page Document | Oct. 26, 2020 |
| Cover page. [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 26, 2020 |
| Entity Registrant Name | Northwest Bancshares, Inc. |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 001-34582 |
| Entity Tax Identification Number | 27-0950358 |
| Entity Address, Address Line One | 100 Liberty Street |
| Entity Address, City or Town | Warren |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 16365 |
| City Area Code | 814 |
| Local Phone Number | 726-2140 |
| Title of 12(b) Security | Common Stock, 0.01 Par Value |
| Trading Symbol | NWBI |
| Security Exchange Name | NASDAQ |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001471265 |
| Amendment Flag | false |
