| Document And Entity Information | Nov. 04, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 4, 2021 |
| Entity Registrant Name | Karuna Therapeutics, Inc. |
| Entity Central Index Key | 0001771917 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-38958 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 27-0605902 |
| Entity Address, Address Line One | 99 High Street, Floor 26 |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02110 |
| City Area Code | 857 |
| Local Phone Number | 449-2244 |
| Entity Information, Former Legal or Registered Name | Not Applicable |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.0001 |
| Trading Symbol | KRTX |
| Security Exchange Name | NASDAQ |
