Document and Entity Information	9 Months Ended
	Sep. 30, 2020 shares
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Trading Symbol	CPTP
Entity Registrant Name	CAPITAL PROPERTIES INC /RI/
Entity Central Index Key	0000202947
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Current Reporting Status	Yes
Entity File Number	001-08499
Entity Tax Identification Number	05-0386287
Entity Address, Address Line One	5 Steeple Street, Unit 303
Entity Address, City or Town	Providence
Entity Address, State or Province	RI
Entity Address, Postal Zip Code	02903
City Area Code	401
Local Phone Number	435-7171
Entity Interactive Data Current	Yes
Title of 12(b) Security	Class A Common Stock
Document Quarterly Report	true
Document Transition Report	false
Entity Common Stock, Shares Outstanding	6,599,912
Entity Incorporation, State or Country Code	RI