Shareholders' Equity - Minimum Capital Requirements were Considered Well Capitalized by FRB and FDIC (Details)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 438,979,000	$ 420,398,000
Tier I capital (to risk-weighted assets), Actual, Amount	403,484,000	384,518,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	403,484,000	384,518,000
Tier I capital (to average assets), Actual, Amount	$ 403,484,000	$ 384,518,000
Total capital (to risk-weighted assets), Actual, Ratio	0.134	0.136
Tier I capital (to risk-weighted assets), Actual, Ratio	0.123	0.124
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	0.123	0.124
Tier I capital (to average assets), Actual, Ratio	0.081	0.083
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 344,617,000	$ 325,617,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	278,976,000	263,595,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	229,745,000	217,078,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 199,142,000	$ 185,015,000
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	$ 422,899,000	$ 409,806,000
Tier I capital (to risk-weighted assets), Actual, Amount	387,404,000	373,926,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	387,404,000	373,926,000
Tier I capital (to average assets), Actual, Amount	$ 387,404,000	$ 373,926,000
Total capital (to risk-weighted assets), Actual, Ratio	0.129	0.132
Tier I capital (to risk-weighted assets), Actual, Ratio	0.118	0.121
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	0.118	0.121
Tier I capital (to average assets), Actual, Ratio	0.078	0.081
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 344,585,000	$ 325,587,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	278,950,000	263,570,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	229,723,000	217,058,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 199,131,000	$ 185,003,000
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	$ 328,176,000	$ 310,082,000
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	262,541,000	248,066,000
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	213,314,000	201,554,000
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 248,913,000	$ 231,254,000
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