Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Nov. 05, 2018
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Trading Symbol SNNA  
Entity Registrant Name Sienna Biopharmaceuticals, Inc.  
Entity Central Index Key 0001656328  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Smaller Reporting Company true  
Emerging Growth Company true  
Entity Extended Transition Period true  
Entity Common Stock, Shares Outstanding   21,140,101