Shareholders' Equity - Minimum Capital Requirements were Considered Well Capitalized by FRB and FDIC (Details)	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 518,398,000	$ 506,239,000
Tier I capital (to risk-weighted assets), Actual, Amount	478,273,000	466,369,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	478,273,000	466,369,000
Tier I capital (to average assets), Actual, Amount	$ 478,273,000	$ 466,369,000
Total capital (to risk-weighted assets), Actual, Ratio	0.142	0.135
Tier I capital (to risk-weighted assets), Actual, Ratio	0.131	0.125
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	0.131	0.125
Tier I capital (to average assets), Actual, Ratio	0.088	0.085
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 383,672,000	$ 393,285,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	310,591,000	318,373,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	255,781,000	262,190,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 216,714,000	$ 219,309,000
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	0.07	0.07
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.04	0.04
Cambridge Trust Company
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 496,819,000	$ 490,175,000
Tier I capital (to risk-weighted assets), Actual, Amount	456,694,000	450,305,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	456,694,000	450,305,000
Tier I capital (to average assets), Actual, Amount	$ 456,694,000	$ 450,305,000
Total capital (to risk-weighted assets), Actual, Ratio	0.136	0.131
Tier I capital (to risk-weighted assets), Actual, Ratio	0.125	0.12
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	0.125	0.12
Tier I capital (to average assets), Actual, Ratio	0.084	0.082
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 383,636,000	$ 393,246,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	310,563,000	318,342,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	255,757,000	262,164,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 216,700,000	$ 219,296,000
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	0.07	0.07
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	0.04	0.04
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	$ 365,368,000	$ 374,520,000
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	292,294,000	299,616,000
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	237,489,000	243,438,000
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 270,875,000	$ 274,120,000
Total capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.10	0.10
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.08	0.08
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	0.05	0.05