Convertible Preferred Stock and Stockholders' Equity (Deficit) - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Jun. 19, 2020	Dec. 31, 2020	Sep. 30, 2020	Jun. 17, 2020	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Convertible preferred stock, shares outstanding		0			207,164,000
Common stock, shares issued		52,117,000			9,073,000
Payment to several nonaccredited investors		$ 0.1
2020 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Common stock reserved for issuance		772,000		2,930,836
Convertible Preferred Stock
Class Of Stock [Line Items]
Convertible preferred stock, shares outstanding		0			207,164,000	207,164,000
Common Stock
Class Of Stock [Line Items]
Conversion of Compass Therapeutics LLC preferred shares into common shares upon consummation of the reverse merger, Shares		30,630,000
Shares of common stock sold		12,096,000
Purchase price per share		$ 5.00	$ 5.00
Common Stock | Private Placement Offering
Class Of Stock [Line Items]
Shares of common stock sold	12,096,442
Purchase price per share	$ 5.00
Common Stock | Olivia Ventures Inc
Class Of Stock [Line Items]
Common stock, shares issued		1,000,000
Common Stock | 2020 Stock Option and Incentive Plan
Class Of Stock [Line Items]
Common stock reserved for issuance		2,900,000