Shareholders' Equity - Minimum Capital Requirements were Considered Well Capitalized by FRB and FDIC (Details)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 401,412,000	$ 378,393,000
Tier I capital (to risk-weighted assets), Actual, Amount	365,434,000	344,409,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	365,434,000	344,409,000
Tier I capital (to average assets), Actual, Amount	$ 365,434,000	$ 344,409,000
Total capital (to risk-weighted assets), Actual, Ratio	0.139	0.139
Tier I capital (to risk-weighted assets), Actual, Ratio	0.126	0.127
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	0.126	0.127
Tier I capital (to average assets), Actual, Ratio	0.087	0.089
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 303,775,000	$ 285,145,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	245,913,000	230,832,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	202,516,000	190,097,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 167,450,000	$ 155,009,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.105	0.105
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.085	0.085
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.070	0.070
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.040	0.040
Cambridge Trust Company
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 396,759,000	$ 376,209,000
Tier I capital (to risk-weighted assets), Actual, Amount	360,781,000	342,229,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	360,781,000	342,229,000
Tier I capital (to average assets), Actual, Amount	$ 360,781,000	$ 342,229,000
Total capital (to risk-weighted assets), Actual, Ratio	0.137	0.139
Tier I capital (to risk-weighted assets), Actual, Ratio	0.125	0.126
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	0.125	0.126
Tier I capital (to average assets), Actual, Ratio	0.086	0.088
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 303,746,000	$ 285,117,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	245,890,000	230,809,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	202,497,000	190,078,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 167,439,000	$ 154,999,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.105	0.105
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.085	0.085
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.070	0.070
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.040	0.040
Cambridge Trust Company | Minimum
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 289,282,000	$ 271,540,000
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	231,426,000	217,232,000
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	188,033,000	176,501,000
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 209,299,000	$ 193,748,000
Total capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.100	0.100
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.080	0.080
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.065	0.065
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.050	0.050