| Document and Entity Information | Mar. 12, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001042074 |
| Document Type | 8-K |
| Document Period End Date | Mar. 12, 2020 |
| Entity Registrant Name | CymaBay Therapeutics, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-36500 |
| Entity Tax Identification Number | 94-3103561 |
| Entity Address, Address Line One | 7575 Gateway Blvd. |
| Entity Address, Address Line Two | Suite 110 |
| Entity Address, City or Town | Newark |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94560 |
| City Area Code | (510) |
| Local Phone Number | 293-8800 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, $0.0001 par value per share |
| Trading Symbol | CBAY |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
