| Document and Entity Information | Dec. 15, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Dec. 15, 2020 |
| Entity Registrant Name | WEX Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-32426 |
| Entity Tax Identification Number | 01-0526993 |
| Entity Address, Address Line One | 1 Hancock Street |
| Entity Address, City or Town | Portland |
| Entity Address, State or Province | ME |
| Entity Address, Postal Zip Code | 04101 |
| City Area Code | 207 |
| Local Phone Number | 773-8171 |
| 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 | 0001309108 |
| Title of 12(b) Security | Common Stock, $0.01 par value |
| Trading Symbol | WEX |
| Security Exchange Name | NYSE |
