Stockholders' Equity - Minimum Required Capital (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Total capital (to risk weighted assets)
Actual - Total Capital to Risk Weighted Assets	$ 408,223	$ 410,088
Actual - Total Capital to Risk Weighted Assets Ratio (percentage)	19.40%	19.70%
Minimum Capital Requirement - Total Capital	$ 167,942	$ 166,635
Minimum Capital Requirement - Total Capital to Risk Weighted Assets Ratio (percentage)	8.00%	8.00%
Minimum to Be Well Capitalized - Total Capital	$ 209,928	$ 208,294
Minimum to Be Well Capitalized - Total Capital to Risk Weighted Assets Ratio (percentage)	10.00%	10.00%
Tier 1 capital (to risk weighted assets)
Actual - Tier 1 Capital to Risk Weighted Assets	$ 388,039	$ 389,211
Actual - Tier 1 Capital to Risk Weighted Assets Ratio (percentage)	18.50%	18.70%
Minimum Capital Requirement - Tier 1 Capital to Risk Weighted Assets	$ 125,957	$ 124,977
Minimum Capital Requirement - Tier 1 Capital to Risk Weighted Assets Ratio (percentage)	6.00%	6.00%
Minimum to Be Well Capitalized - Tier 1 Capital to Risk Weighted Assets	$ 167,942	$ 166,635
Minimum to Be Well Capitalized - Tier 1 Capital to Risk Weighted Assets Ratio (percentage)	8.00%	8.00%
Common equity Tier 1 (to risk weighted assets)
Actual - Common Equity to Risk Weighted Assets	$ 388,039	$ 389,211
Actual - Common Equity to Risk Weighted Assets Ratio (percentage)	18.50%	18.70%
Minimum Capital Requirement - Common Equity to Risk Weighted Assets	$ 94,468	$ 93,732
Minimum Capital Requirement - Common Equity to Risk Weighted Assets Ratio (percentage)	4.50%	4.50%
Minimum to Be Well Capitalized - Common Equity to Risk Weighted Assets	$ 136,453	$ 135,391
Minimum to Be Well Capitalized - Common Equity to Risk Weighted Assets Ratio (percentage)	6.50%	6.50%
Tier 1 capital (to average assets)
Actual - Tier 1 Capital to Average Assets	$ 388,039	$ 389,211
Actual - Tier 1 Capital to Average Assets Ratio (percentage)	14.70%	14.90%
Minimum Capital Requirement - Tier 1 Capital to Average Assets	$ 105,730	$ 104,278
Minimum Capital Requirement - Tier 1 Capital to Average Assets Ratio (percentage)	4.00%	4.00%
Minimum to Be Well Capitalized - Tier 1 Capital to Average Assets	$ 132,162	$ 130,348
Minimum to Be Well Capitalized - Tier 1 Capital to Average Assets Ratio (percentage)	5.00%	5.00%
Blue Hills Bank
Total capital (to risk weighted assets)
Actual - Total Capital to Risk Weighted Assets	$ 349,045	$ 341,175
Actual - Total Capital to Risk Weighted Assets Ratio (percentage)	16.60%	16.40%
Minimum Capital Requirement - Total Capital	$ 167,751	$ 166,391
Minimum Capital Requirement - Total Capital to Risk Weighted Assets Ratio (percentage)	8.00%	8.00%
Minimum to Be Well Capitalized - Total Capital	$ 209,689	$ 207,988
Minimum to Be Well Capitalized - Total Capital to Risk Weighted Assets Ratio (percentage)	10.00%	10.00%
Tier 1 capital (to risk weighted assets)
Actual - Tier 1 Capital to Risk Weighted Assets	$ 328,860	$ 320,298
Actual - Tier 1 Capital to Risk Weighted Assets Ratio (percentage)	15.70%	15.40%
Minimum Capital Requirement - Tier 1 Capital to Risk Weighted Assets	$ 125,813	$ 124,793
Minimum Capital Requirement - Tier 1 Capital to Risk Weighted Assets Ratio (percentage)	6.00%	6.00%
Minimum to Be Well Capitalized - Tier 1 Capital to Risk Weighted Assets	$ 167,751	$ 166,391
Minimum to Be Well Capitalized - Tier 1 Capital to Risk Weighted Assets Ratio (percentage)	8.00%	8.00%
Common equity Tier 1 (to risk weighted assets)
Actual - Common Equity to Risk Weighted Assets	$ 328,860	$ 320,298
Actual - Common Equity to Risk Weighted Assets Ratio (percentage)	15.70%	15.40%
Minimum Capital Requirement - Common Equity to Risk Weighted Assets	$ 94,360	$ 93,595
Minimum Capital Requirement - Common Equity to Risk Weighted Assets Ratio (percentage)	4.50%	4.50%
Minimum to Be Well Capitalized - Common Equity to Risk Weighted Assets	$ 136,298	$ 135,192
Minimum to Be Well Capitalized - Common Equity to Risk Weighted Assets Ratio (percentage)	6.50%	6.50%
Tier 1 capital (to average assets)
Actual - Tier 1 Capital to Average Assets	$ 328,860	$ 320,298
Actual - Tier 1 Capital to Average Assets Ratio (percentage)	12.50%	12.30%
Minimum Capital Requirement - Tier 1 Capital to Average Assets	$ 105,603	$ 104,137
Minimum Capital Requirement - Tier 1 Capital to Average Assets Ratio (percentage)	4.00%	4.00%
Minimum to Be Well Capitalized - Tier 1 Capital to Average Assets	$ 132,004	$ 130,171
Minimum to Be Well Capitalized - Tier 1 Capital to Average Assets Ratio (percentage)	5.00%	5.00%

Common Equity Risk Based Capital [Abstract]

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The amount of total capital as defined in the regulations.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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