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  
Trading Symbol BL  
Entity Registrant Name BLACKLINE, INC.  
Entity Central Index Key 0001666134  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   55,639,281
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Shell Company false  
Entity File Number 001-37924  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 46-3354276  
Entity Address, Address Line One 21300 Victory Boulevard  
Entity Address, Address Line Two 12th Floor  
Entity Address, City or Town Woodland Hills  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 91367  
City Area Code 818  
Local Phone Number 223-9008  
Document Quarterly Report true  
Document Transition Report false  
Title of 12(b) Security Common stock, par value $0.01 per share  
Security Exchange Name NASDAQ