v3.19.1
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2018
Mar. 08, 2019
Jun. 30, 2018
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Trading Symbol SNNA    
Entity Registrant Name Sienna Biopharmaceuticals, Inc.    
Entity Central Index Key 0001656328    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Accelerated Filer    
Entity Common Stock, Shares Outstanding   30,375,820  
Entity Public Float     $ 0
Emerging Growth Company true    
Entity Small Business true    
Entity Shell Company false    
Entity Extended Transition Period true