Nature of Operations and Basis of Presentation - Additional Information (Details) - USD ($)					9 Months Ended
	Jul. 24, 2020	Apr. 30, 2020	Mar. 18, 2020	Sep. 19, 2017	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Organization Consolidation and Presentation of Financial Statement [Line Items]
Accumulated deficit					$ 92,898,278		$ 76,250,512
Cash and cash equivalents					6,157,448		$ 12,077,640
Net cash used in operating activities					13,147,533	$ 25,515,573
Proceeds from issuance of common stock, net of issuance costs					$ 6,667,091
Common stock, shares issued					11,912,731		10,095,176
Current liabilities					$ 4,489,424		$ 2,768,913
Merger of Private Acer with opexa therapeutics incorporation and 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
Merger and Reincorporation
Organization Consolidation and Presentation of Financial Statement [Line Items]
Merger incorporation description					On May 15, 2018, the Company changed its state of incorporation from the State of Texas to the State of Delaware pursuant to a plan of conversion, dated May 15, 2018. Immediately following the reincorporation, the holding company structure was eliminated by merging wholly-owned subsidiary Private Acer with and into the Company. The Company was the surviving corporation in connection with the subsidiary merger.
Date of reincorporation					May 15, 2018
Lincoln Park
Organization Consolidation and Presentation of Financial Statement [Line Items]
Shares value committed to be issued under agreement		$ 15,000,000
Expected increase in maximum number of common stock shares may sell in single business day on regular purchases		100,000
Maximum number of common stock shares may sell in single business day on regular purchases		50,000
Market price of common stock at the time of sale and subject to maximum limit per regular purchase		$ 1,000,000
Issuance of common stock for commitment fee, shares		148,148
Fair value of the commitment fee shares, recorded to general and administrative expense along with other costs		$ 400,000
Cost, Product and Service [Extensible List]		us-gaap:GeneralAndAdministrativeExpenseMember
Common stock, shares issued					0
Maximum | Lincoln Park
Organization Consolidation and Presentation of Financial Statement [Line Items]
Shares value committed to be issued under agreement		$ 15,000,000
At-the-Market Facility
Organization Consolidation and Presentation of Financial Statement [Line Items]
Issuance of common stock					1,418,551
Proceeds from issuance of common stock gross					$ 6,100,000
Common stock offering costs					300,000
Proceeds from issuance of common stock, net of issuance costs					$ 5,800,000
At-the-Market Facility | Weighted Average
Organization Consolidation and Presentation of Financial Statement [Line Items]
Issuance of common stock, price per share					$ 4.2664
At-the-Market Facility | Maximum
Organization Consolidation and Presentation of Financial Statement [Line Items]
Aggregate offering price of common stock			$ 50,000,000
Private Placement | Directors, Officers and Employees
Organization Consolidation and Presentation of Financial Statement [Line Items]
Issuance of common stock	244,998
Issuance of common stock, price per share	$ 3.50
Proceeds from issuance of common stock, net of issuance costs	$ 900,000