| Document and Entity Information Document | Nov. 05, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 5, 2021 |
| Entity Registrant Name | Nextdoor Holdings, Inc. |
| Entity Central Index Key | 0001846069 |
| Amendment Flag | false |
| Current Fiscal Year End Date | --12-31 |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-40246 |
| Entity Tax Identification Number | 86-1776836 |
| Entity Address, Address Line One | 420 Taylor Street |
| Entity Address, City or Town | San Francisco, |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94102 |
| City Area Code | (415) |
| Local Phone Number | 344-0333 |
| Written Communications | true |
| Soliciting Material | true |
| Pre-commencement Tender Offer | true |
| Pre-commencement Issuer Tender Offer | true |
| Title of 12(b) Security | Class A common stock, par value $0.0001per share |
| Trading Symbol | KIND |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
