| Document and Entity Information Document | Jan. 19, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 19, 2021 |
| Entity Incorporation, State or Country Code | PA |
| Entity File Number | 001-31940 |
| Entity Tax Identification Number | 25-1255406 |
| Entity Address, Address Line One | One North Shore Center, |
| Entity Address, Address Line Two | 12 Federal Street, |
| Entity Address, City or Town | Pittsburgh, |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 15212 |
| City Area Code | 800 |
| Local Phone Number | 555-5455 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Registrant Name | FNB CORP/PA/ |
| Entity Central Index Key | 0000037808 |
| Amendment Flag | false |
| Common Stock, par value $0.01 per share | New York Stock Exchange | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | FNB |
| Security Exchange Name | NYSE |
| Depositary Shares each representing 1/40th interest in a share of Fixed-to-Floating Rate Non-Cumulative Perpetual Preferred Stock, Series E | New York Stock Exchange | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Depositary Shares each representing 1/40th interest in a share of Fixed-to-Floating Rate Non-Cumulative Perpetual Preferred Stock, Series E |
| Trading Symbol | FNBPrE |
| Security Exchange Name | NYSE |
