Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 31, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	CATC
Entity Registrant Name	CAMBRIDGE BANCORP
Entity Central Index Key	0000711772
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		4,851,047
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-38184
Entity Tax Identification Number	042777442
Entity Address, Address Line One	1336 Massachusetts Avenue
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02138
City Area Code	(617)
Local Phone Number	876-5500