v3.10.0.1
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Oct. 22, 2018
Class of Stock [Line Items]    
Entity Registrant Name Bandwidth Inc.  
Entity Central Index Key 0001514416  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Document Type 10-Q  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Small Business false  
Common Class A    
Class of Stock [Line Items]    
Entity Common Stock, Shares Outstanding   12,655,226
Common Class B    
Class of Stock [Line Items]    
Entity Common Stock, Shares Outstanding   6,699,691