| Document and Entity Information | May 06, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001771917 |
| Document Type | 8-K |
| Document Period End Date | May 6, 2021 |
| Entity Registrant Name | KARUNA THERAPEUTICS, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38958 |
| Entity Tax Identification Number | 27-0605902 |
| Entity Address, Address Line One | 99 High Street |
| Entity Address, Address Line Two | 26th Floor |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, par value $0.0001 |
| Trading Symbol | KRTX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
