| Document and Entity Information | Oct. 28, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001393612 |
| Document Type | 8-K |
| Document Period End Date | Oct. 28, 2020 |
| Entity Registrant Name | DISCOVER FINANCIAL SERVICES |
| Entity File Number | 001-33378 |
| Entity Incorporation State Country Code | DE |
| Entity Tax Identification Number | 36-2517428 |
| Entity Address, Address Line One | 2500 Lake Cook Road |
| Entity Address, City or Town | Riverwoods |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60015 |
| City Area Code | (224) |
| Local Phone Number | 405-0900 |
| 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 | DFS |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
