Condensed Financial Statements of Parent Company - Condensed Statements of Cash Flows (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
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 7,109	$ 7,676	$ 4,272	$ 6,198	$ 5,306	$ 6,659	$ 6,111	$ 5,805	$ 25,257	$ 23,881	$ 14,816
Adjustments to reconcile net income to net cash provided by operating activities:
Change in other assets, net									(11,667)	(12,231)	(758)
Net cash provided by operating activities									28,793	24,022	29,767
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash (used in) provided by investing activities									(113,472)	(226,883)	(61,760)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from the issuance of common stock									38,202
Cash dividends paid on common stock									(9,517)	(8,041)	(7,582)
Net cash provided by (used in) financing activities									127,541	117,743	81,534
Net (decrease)/increase in cash and cash equivalents									42,862	(85,118)	49,541
Cash and cash equivalents at beginning of period				18,473				103,591	18,473	103,591	54,050
Cash and cash equivalents at end of period	61,335				18,473				61,335	18,473	103,591
Cambridge Bancorp [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									25,257	23,881	14,816
Adjustments to reconcile net income to net cash provided by operating activities:
Change in other assets, net									(13)
Undistributed income of subsidiary									(14,621)	(15,350)	(6,764)
Net cash provided by operating activities									10,623	8,531	8,052
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash paid in business combinations									(3,525)
Investment in subsidiary									(38,202)
Net cash (used in) provided by investing activities									(41,727)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from the issuance of common stock									38,576	761	1,522
Repurchase of common stock									(687)	(574)	(470)
Cash dividends paid on common stock									(9,517)	(8,041)	(7,582)
Net cash provided by (used in) financing activities									28,372	(7,854)	(6,530)
Net (decrease)/increase in cash and cash equivalents									(2,732)	677	1,522
Cash and cash equivalents at beginning of period				$ 4,412				$ 3,735	4,412	3,735	2,213
Cash and cash equivalents at end of period	$ 1,680				$ 4,412				$ 1,680	$ 4,412	$ 3,735