Condensed Financial Statements of Parent Company - Condensed Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest expense
Total interest expense	$ 4,807	$ 5,285	$ 4,694	$ 2,857	$ 1,975	$ 1,431	$ 1,082	$ 979	$ 17,643	$ 5,467	$ 3,587
Income tax benefit	2,673	2,708	1,540	1,740	1,585	2,105	1,901	1,616	8,661	7,207	13,358
Income of parent company									25,257	23,881	14,816
Net income	$ 7,109	$ 7,676	$ 4,272	$ 6,198	$ 5,306	$ 6,659	$ 6,111	$ 5,805	25,257	23,881	14,816
Cambridge Bancorp [Member]
Interest and dividend income
Dividends from subsidiary									10,732	8,615	8,052
Total income									10,732	8,615	8,052
Interest expense
Other expenses									132	116
Total interest expense									132	116
Income before income taxes and equity in undistributed income of subsidiary									10,600	8,499	8,052
Income tax benefit									(36)	(32)
Income of parent company									10,636	8,531	8,052
Equity in undistributed income of subsidiary									14,621	15,350	6,764
Net income									$ 25,257	$ 23,881	$ 14,816