Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Convertible Preferred Stock
Temporary equity, Beginning Balance at Dec. 31, 2018					$ 129,870
Temporary equity, Beginning Balance, Shares at Dec. 31, 2018					207,164,000
Beginning Balance at Dec. 31, 2018	$ (84,774)	$ 1	$ 2,389	$ (87,164)
Beginning Balance, Shares at Dec. 31, 2018		6,355,000
Vesting of share-based awards		679,000
Stock-based compensation expense	915		915
Net loss	(34,744)			(34,744)
Temporary equity, Ending Balance at Dec. 31, 2019	$ 129,870				$ 129,870
Temporary equity, Ending Balance, Shares at Dec. 31, 2019	207,164,000				207,164,000
Ending Balance at Dec. 31, 2019	$ (118,603)	$ 1	3,304	(121,908)
Ending Balance, Shares at Dec. 31, 2019		7,034,000
Common shares issued to former shareholders of Olivia Ventures Inc., Shares		1,000,000
Conversion of Compass Therapeutics LLC preferred shares into common shares upon consummation of the reverse merger	129,870	$ 3	129,867
Temporary equity, Conversion of Compass Therapeutics LLC preferred shares into common shares upon consummation of the reverse merger, Shares					(207,164,000)
Temporary equity, Conversion of Compass Therapeutics LLC preferred shares into common shares upon consummation of the reverse merger					$ (129,870)
Conversion of Compass Therapeutics LLC preferred shares into common shares upon consummation of the reverse merger, Shares		30,630,000
Common shares issued in private placement, net of issuance costs of $6.3 million	54,231	$ 1	54,230
Common shares issued in private placement, net of issuance costs of $6.3 million, Shares		12,096,000
Payment to non-participating Compass Therapeutics LLC members upon consummation of Merger	(69)		(69)
Payment to non-participating Compass Therapeutics LLC members upon consummation of Merger, Shares		(13,000)
Vesting of share-based awards		474,000
Stock-based compensation expense	4,016		4,016
Net loss	$ (29,500)			(29,500)
Temporary equity, Ending Balance, Shares at Dec. 31, 2020	0				0
Ending Balance at Dec. 31, 2020	$ 39,945	$ 5	$ 191,348	$ (151,408)
Ending Balance, Shares at Dec. 31, 2020		51,221,000