Cover Page - shares	6 Months Ended
	Jun. 30, 2020	Sep. 18, 2020
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Entity Registrant Name	Checkmate Pharmaceuticals, Inc.
Entity Central Index Key	0001651431
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	No
Entity Shell Company	false
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Incorporation, State or Country Code	DE
Entity Address, State or Province	DE
Entity Address, Country	MA
Title of 12(b) Security	Common Stock
Trading Symbol	CMPI
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		21,546,591