| Cover | Oct. 12, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 12, 2020 |
| Entity Registrant Name | BANDWIDTH INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38285 |
| Entity Tax Identification Number | 56-2242657 |
| Entity Address, Address Line One | 900 Main Campus Drive |
| Entity Address, City or Town | Raleigh |
| Entity Address, State or Province | NC |
| Entity Address, Postal Zip Code | 27606 |
| City Area Code | (800) |
| Local Phone Number | 808-5150 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Class A Common Stock, par value $0.001 per share |
| Trading Symbol | BAND |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001514416 |
