Shareholders' Equity - Minimum Capital Requirements were Considered Well Capitalized by FRB and FDIC (Details) - USD ($)	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Jan. 01, 2015
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 272,727,000		$ 189,888,000
Tier I capital (to risk-weighted assets), Actual, Amount	254,497,000		173,070,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	254,497,000		173,070,000
Tier I capital (to average assets), Actual, Amount	$ 254,497,000		$ 173,070,000
Total capital (to risk-weighted assets), Actual, Ratio	13.60%		13.20%
Tier I capital (to risk-weighted assets), Actual, Ratio	12.70%		12.10%
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	12.70%		12.10%
Tier I capital (to average assets), Actual, Ratio	9.00%		8.50%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 210,342,000		$ 114,666,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	170,277,000		86,000,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	140,228,000		64,500,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 113,365,000		$ 81,507,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	10.50%		8.00%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	8.50%		6.00%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	7.00%		4.50%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	4.00%		4.00%
Minimum
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Ratio	10.50%			8.00%
Tier I capital (to risk-weighted assets), Actual, Ratio	8.50%			6.00%
Tier I capital (to average assets), Actual, Ratio		9.00%		4.00%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount			$ 141,541,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount			112,875,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount			91,375,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount			$ 81,507,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio			9.875%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio			7.875%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio			6.375%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio			4.00%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount			$ 150,500,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount			121,833,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount			100,333,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount			$ 81,507,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio			10.50%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio			8.50%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio			7.00%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio			4.00%
Cambridge Trust Company
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 271,034,000		$ 185,507,000
Tier I capital (to risk-weighted assets), Actual, Amount	252,804,000		168,689,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	252,804,000		168,689,000
Tier I capital (to average assets), Actual, Amount	$ 252,804,000		$ 168,689,000
Total capital (to risk-weighted assets), Actual, Ratio	13.50%		12.90%
Tier I capital (to risk-weighted assets), Actual, Ratio	12.60%		11.80%
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	12.60%		11.80%
Tier I capital (to average assets), Actual, Ratio	8.90%		8.30%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 210,341,000		$ 114,666,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	170,276,000		86,000,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	140,227,000		64,500,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Amount	$ 113,364,000		$ 81,507,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	10.50%		8.00%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	8.50%		6.00%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	7.00%		4.50%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer, Ratio	4.00%		4.00%
Cambridge Trust Company | Minimum
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount			$ 141,541,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount			112,875,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount			91,375,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount			$ 81,507,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio			9.875%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio			7.875%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio			6.375%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio			4.00%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount			$ 150,500,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount			121,833,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount			100,333,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount			$ 81,507,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio			10.50%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio			8.50%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio			7.00%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio			4.00%
Total capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 200,325,000		$ 143,333,000
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	160,260,000		114,666,000
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	130,211,000		93,166,000
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 141,705,000		$ 101,884,000
Total capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%		10.00%
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%		8.00%
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%		6.50%
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%		5.00%