| Cover Page - USD ($) | 12 Months Ended | ||
| Dec. 31, 2019 | Feb. 29, 2020 | Jun. 28, 2019 | |
| Document Information [Line Items] | |||
| Document Type | 10-K/A | ||
| Amendment Flag | true | ||
| Document Period End Date | Dec. 31, 2019 | ||
| Document Fiscal Year Focus | 2019 | ||
| Document Fiscal Period Focus | FY | ||
| Trading Symbol | CBAY | ||
| Entity Registrant Name | CymaBay Therapeutics, Inc. | ||
| Entity Central Index Key | 0001042074 | ||
| Current Fiscal Year End Date | --12-31 | ||
| Entity Well-known Seasoned Issuer | No | ||
| Entity Filer Category | Accelerated Filer | ||
| Entity Current Reporting Status | Yes | ||
| Entity Voluntary Filers | No | ||
| Entity Small Business | true | ||
| Entity Emerging Growth Company | false | ||
| Entity Common Stock, Shares Outstanding | 68,882,459 | ||
| Entity Shell Company | false | ||
| 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 | ||
| Entity Interactive Data Current | Yes | ||
| Title of 12(b) Security | Common stock, $0.0001 par value per share | ||
| Security Exchange Name | NASDAQ | ||
| Entity Incorporation, State or Country Code | DE | ||
| Document Annual Report | true | ||
| Document Transition Report | false | ||
| Entity Public Float | $ 487,988,477 | ||
| Amendment Description | CymaBay Therapeutics, Inc. (“CymaBay,” “the Company,” “we,” “us,” or “our”) filed its Annual Report on Form 10-K for the fiscal year ended December 31, 2019 (the “Form 10-K”) with the U.S. Securities and Exchange Commission (the “SEC”) on March 16, 2020. The Company is filing this Amendment No. 1 on Form 10-K/A (the “Form 10-K/A”) for the purpose of including in Part III the information that was to be incorporated by reference from its definitive proxy statement for the 2020 annual meeting of stockholders. This Form 10-K/A amends and restates in their entirety Items 10 through 14 of Part III of the Form 10-K. In addition, the reference on the cover of the Form 10-K to the incorporation by reference of the Company’s Proxy Statement for its 2020 Annual Meeting of Stockholders into Part III of the Form 10-K is hereby amended to delete that reference. In addition, Item 15 of Part IV of the Form 10-K is amended and restated in its entirety to file new certifications by our principal executive officer and principal financial officer as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended.Except as expressly noted in this Form 10-K/A, this Form 10-K/A does not reflect events occurring after the original filing of the Form 10-K or modify or update in any way any of the other disclosures contained in the Form 10-K including, without limitation, the financial statements. Accordingly, this Form 10-K/A should be read in conjunction with the Form 10-K and the Company’s other filings with the SEC. |
