| Document and Entity Information | Sep. 02, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001471265 |
| Document Type | 8-K |
| Document Period End Date | Sep. 2, 2020 |
| Entity Registrant Name | NORTHWEST BANCSHARES, INC. |
| Entity Incorporation State 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | NWBI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
