Summary of Significant Accounting Policies - Additional Information (Details)			12 Months Ended
	Jan. 01, 2021 USD ($)	Jun. 17, 2020 $ / shares shares	Dec. 31, 2020 USD ($) Segment $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Sep. 30, 2020 $ / shares	Mar. 31, 2020 $ / shares shares	Jul. 01, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Stock conversion ratio description			Upon the closing of the Merger, the common and convertible preferred units of Compass Therapeutics LLC were converted into the Company’s common stock. The Company effected a 0.15-for-one stock conversion ratio for its issued and outstanding convertible preferred units. Common units were converted using the same ratio after factoring in the relevant strike price of each grant.
Common units outstanding | shares			0
Convertible preferred units outstanding | shares			0
Preferred stock, shares authorized | shares		5,000,000	10,000,000	10,000,000		10,000,000
Preferred stock, par value | $ / shares		$ 0.0001	$ 0.0001	$ 0.0001		$ 0.0001
Common stock, shares authorized | shares		50,000,000	300,000,000	300,000,000		300,000,000
Common stock, par value | $ / shares		$ 0.0001	$ 0.0001	$ 0.0001		$ 0.0001
Number of operating segments | Segment			1
Restricted cash			$ 200,000	$ 200,000
Letter of credit							$ 100,000
Impairment losses on long-lived assets			0	0
Expected dividend yield			0
Income tax expense			$ 32,000	91,000
ASU 2016-02 | Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2021
Change in accounting principle, accounting standards update, transition option elected [Extensible List]	us-gaap:AccountingStandardsUpdate201602RetrospectiveMember
Operating lease ROU assets	$ 5,100,000
Operating lease liabilities	$ 5,100,000
ASU 2018-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted			true
Change in accounting principle, accounting standards update, adoption date			Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect			true
Compass Therapeutics LLC
Summary Of Significant Accounting Policies [Line Items]
Income tax expense			$ 0	0
Common Stock
Summary Of Significant Accounting Policies [Line Items]
Purchase price per share | $ / shares			$ 5.00		$ 5.00
Money Market Funds
Summary Of Significant Accounting Policies [Line Items]
Cash equivalents			$ 43,600,000	$ 22,800,000
Convertible Preferred Units
Summary Of Significant Accounting Policies [Line Items]
Stock conversion ratio		0.15
Common Units
Summary Of Significant Accounting Policies [Line Items]
Stock conversion ratio		0.15