Nature of Operations and Basis of Presentation - Additional Information (Details)		9 Months Ended
	Sep. 19, 2017	Sep. 30, 2018 Patient $ / shares	May 15, 2018 $ / shares	May 14, 2018 $ / shares	Dec. 31, 2017 $ / shares
Organization Consolidation and Presentation of Financial Statement [Line Items]
Number of patients impacted | Patient		7,000
Common stock, par value		$ 0.0001		$ 0.01	$ 0.01
Merger of Private Acer with Opexa Merger Sub Incorporation
Organization Consolidation and Presentation of Financial Statement [Line Items]
Business combination date of completion	Sep. 19, 2017
Business combination date of agreement	Jun. 30, 2017
Reverse stock split ratio	0.09656678989
Delaware Reincorporation and Subsidiary Merger
Organization Consolidation and Presentation of Financial Statement [Line Items]
State of incorporation description		On May 15, 2018, the Company changed its state of incorporation from the State of Texas to the State of Delaware (the “Reincorporation”) pursuant to a plan of conversion, dated May 15, 2018. The Company filed the following instruments on May 15, 2018 to effect the Reincorporation: (i) a certificate of conversion with the Texas Secretary of State; (ii) a certificate of conversion with the Delaware Secretary of State; and (iii) a certificate of incorporation with the Delaware Secretary of State. Pursuant to the plan of conversion, the Company also adopted new bylaws, which became effective with the Reincorporation. The Reincorporation was approved by the Company’s stockholders at its annual meeting on May 14, 2018. Immediately following the Reincorporation, the Company eliminated its holding company structure by merging its wholly-owned subsidiary Private Acer with and into the Company (the “Subsidiary Merger”). The Company was the surviving corporation in connection with the Subsidiary Merger. As the Delaware certificate of incorporation used the placeholder name of “Acer Reincorporation, Inc.” due to “Acer Therapeutics Inc.” already being in existence in Delaware as Private Acer, in connection with the Subsidiary Merger the Company changed its name to “Acer Therapeutics Inc.” pursuant to a certificate of ownership and merger filed with the Delaware Secretary of State on May 15, 2018. As a result of the reincorporation, the par value of the Company’s common stock was reduced to $0.0001 from $0.01.
Date Of Reincorporation		May 15, 2018
Common stock, par value			$ 0.0001