Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Oct. 31, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Title of 12(b) Security Common Stock, $0.001 par value per share  
Trading Symbol SPRO  
Security Exchange Name NASDAQ  
Entity Registrant Name Spero Therapeutics, Inc.  
Entity Central Index Key 0001701108  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   19,128,146
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Shell Company false  
Entity File Number 001-38266  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 46-4590683  
Entity Address, Address Line One 675 Massachusetts Avenue,  
Entity Address, Address Line Two 14th Floor  
Entity Address, City or Town Cambridge  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02139  
City Area Code 857  
Local Phone Number 242-1600  
Document Quarterly Report true  
Document Transition Report false