Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Nov. 04, 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  
Trading Symbol EIGR  
Entity Registrant Name Eiger BioPharmaceuticals, Inc.  
Entity Central Index Key 0001305253  
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   24,495,047
Entity Current Reporting Status Yes  
Entity Shell Company false  
Entity File Number 001-36183  
Entity Tax Identification Number 33-0971591  
Entity Address, Address Line One 2155 Park Boulevard  
Entity Address, City or Town Palo Alto  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94306  
City Area Code 650  
Local Phone Number 272-6138  
Entity Interactive Data Current Yes  
Title of 12(b) Security Common Stock (par value $0.001 per share)  
Security Exchange Name NASDAQ  
Entity Incorporation, State or Country Code DE  
Document Quarterly Report true  
Document Transition Report false