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Cover - shares
 9 Months Ended
Sep. 30, 2021
Oct. 28, 2021
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2021  
Document Transition Report false  
Entity File Number 001-32426  
Entity Registrant Name WEX Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 01-0526993  
Entity Address, Address Line One 1 Hancock St.,  
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  
Title of 12(b) Security Common Stock, $0.01 par value  
Trading Symbol WEX  
Security Exchange Name NYSE  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   44,818,545
Entity Central Index Key 0001309108  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2021  
Document Fiscal Period Focus Q3  
Amendment Flag false