| Document and Entity Information | Oct. 02, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001666134 |
| Document Type | 8-K |
| Document Period End Date | Oct. 2, 2020 |
| Entity Registrant Name | BLACKLINE, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-37924 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.01, par value |
| Trading Symbol | BL |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
