Shareholders' Equity - Minimum Capital Requirements were Considered Well Capitalized by FRB and FDIC (Details) - USD ($)	Mar. 31, 2019		Dec. 31, 2018
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 193,883,000		$ 189,888,000
Tier I capital (to risk-weighted assets), Actual, Amount	177,181,000		173,070,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	177,181,000		173,070,000
Tier I capital (to average assets), Actual, Amount	$ 177,181,000		$ 173,070,000
Total capital (to risk-weighted assets), Actual, Ratio	13.20%		13.20%
Tier I capital (to risk-weighted assets), Actual, Ratio	12.10%		12.10%
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	12.10%		12.10%
Tier I capital (to average assets), Actual, Ratio	8.30%		8.50%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	$ 117,145,000		$ 114,666,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	87,859,000		86,000,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	65,894,000		64,500,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy, Amount	$ 85,421,000		$ 81,507,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	8.00%		8.00%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	6.00%		6.00%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	4.50%		4.50%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy, Ratio	4.00%		4.00%
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	$ 153,753,000	[1]	$ 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	124,467,000	[1]	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	102,502,000	[1]	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	$ 85,421,000	[1]	$ 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%	[1]	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%	[1]	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%	[1]	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%	[1]	4.00%
Cambridge Trust Company
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 186,489,000		$ 185,507,000
Tier I capital (to risk-weighted assets), Actual, Amount	169,787,000		168,689,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	169,787,000		168,689,000
Tier I capital (to average assets), Actual, Amount	$ 169,787,000		$ 168,689,000
Total capital (to risk-weighted assets), Actual, Ratio	12.70%		12.90%
Tier I capital (to risk-weighted assets), Actual, Ratio	11.60%		11.80%
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	11.60%		11.80%
Tier I capital (to average assets), Actual, Ratio	8.00%		8.30%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	$ 117,145,000		$ 114,666,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	87,859,000		86,000,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	65,894,000		64,500,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy, Amount	$ 85,421,000		$ 81,507,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	8.00%		8.00%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	6.00%		6.00%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	4.50%		4.50%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy, 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	$ 153,753,000	[1]	$ 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	124,467,000	[1]	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	102,502,000	[1]	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	$ 85,421,000	[1]	$ 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%	[1]	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%	[1]	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%	[1]	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%	[1]	4.00%
Total capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 146,431,000		$ 143,333,000
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	117,145,000		114,666,000
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	95,180,000		93,166,000
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 106,776,000		$ 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%

[1] The 2013 Capital Rules adopted by the Federal Reserve, Office of the Comptroller of the Currency and the Federal Deposit Insurance Corporation implementing Basel III were fully phased-in effective January 1, 2019.