Condensed Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Convertible Preferred Stock
Beginning Balance at Dec. 31, 2019	$ (118,603)	$ 1	$ 3,304	$ (121,908)
Temporary equity, Beginning Balance, Shares at Dec. 31, 2019					207,164
Temporary equity, Beginning Balance at Dec. 31, 2019					$ 129,870
Beginning Balance, Shares at Dec. 31, 2019		7,034
Vesting of share-based awards		88
Stock-based compensation	247		247
Net loss	(6,402)			(6,402)
Ending Balance at Mar. 31, 2020	(124,758)	$ 1	3,551	(128,310)
Temporary equity, Ending Balance, Shares at Mar. 31, 2020					207,164
Temporary equity, Ending Balance at Mar. 31, 2020					$ 129,870
Ending Balance, Shares at Mar. 31, 2020		7,122
Vesting of share-based awards		62
Stock-based compensation	183		183
Common shares issued, net of issuance costs of $6,902		12,097
Common shares issued, net of issuance costs of $6,902	53,581	$ 1	53,580
Common shares issued for acquisitions, Shares		1,000
Temporary equity, Conversion of Compass Therapeutics LLC preferred into common shares, Shares					(207,164)
Temporary equity, Conversion of Compass Therapeutics LLC preferred into common shares					$ (129,870)
Conversion of compass Therapeutics LLC preferred into common shares	129,870	$ 3	129,867
Conversion of compass Therapeutics LLC preferred into common shares, Shares		30,630
Payment to non-participating Compass Therapeutics LLC members	(69)		(69)
Payment to non-participating Compass Therapeutics LLC members, Shares		(14)
Net loss	(5,557)			(5,557)
Ending Balance at Jun. 30, 2020	53,250	$ 5	187,112	(133,867)
Temporary equity, Ending Balance, Shares at Jun. 30, 2020					0
Temporary equity, Ending Balance at Jun. 30, 2020					$ 0
Ending Balance, Shares at Jun. 30, 2020		50,897
Vesting of share-based awards		127
Stock-based compensation	2,536		2,536
Adjustment to issuance costs related to June 2020 private placement	651		651
Net loss	(9,150)			(9,150)
Ending Balance at Sep. 30, 2020	47,287	$ 5	190,299	(143,017)
Temporary equity, Ending Balance, Shares at Sep. 30, 2020					0
Temporary equity, Ending Balance at Sep. 30, 2020					$ 0
Ending Balance, Shares at Sep. 30, 2020		51,024
Beginning Balance at Dec. 31, 2020	39,945	$ 5	191,348	(151,408)
Temporary equity, Beginning Balance, Shares at Dec. 31, 2020					0
Temporary equity, Beginning Balance at Dec. 31, 2020					$ 0
Beginning Balance, Shares at Dec. 31, 2020		51,221
Vesting of share-based awards		92
Stock-based compensation	948		948
Net loss	(7,422)			(7,422)
Ending Balance at Mar. 31, 2021	33,471	$ 5	192,296	(158,830)
Temporary equity, Ending Balance, Shares at Mar. 31, 2021					0
Temporary equity, Ending Balance at Mar. 31, 2021					$ 0
Ending Balance, Shares at Mar. 31, 2021		51,313
Vesting of share-based awards		88
Stock-based compensation	908		908
Common shares issued for acquisitions	50,300	$ 1	50,299
Common shares issued for acquisitions, Shares		10,265
Net loss	(55,804)			(55,804)
Ending Balance at Jun. 30, 2021	28,875	$ 6	243,503	(214,634)
Temporary equity, Ending Balance, Shares at Jun. 30, 2021					0
Temporary equity, Ending Balance at Jun. 30, 2021					$ 0
Ending Balance, Shares at Jun. 30, 2021		61,666
Vesting of share-based awards		94
Stock-based compensation	987		987
Net loss	(5,975)			(5,975)
Ending Balance at Sep. 30, 2021	$ 23,888	$ 6	$ 244,490	$ (220,608)
Temporary equity, Ending Balance, Shares at Sep. 30, 2021					0
Temporary equity, Ending Balance at Sep. 30, 2021					$ 0
Ending Balance, Shares at Sep. 30, 2021		61,760