Regulatory Capital Requirements (Details) $ in Thousands	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Total Capital (to Risk-Weighted Assets)
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 871,043	$ 814,505
Total Capital (to Risk-Weighted Assets), Actual Ratio	0.136	0.174
Total Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Amount	$ 512,904	$ 374,650
Total Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Ratio	0.08	0.08
Total Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Amount	$ 641,130	$ 468,313
Total Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Ratio	0.10	0.10
Tier I Capital (to Risk-Weighted Assets)
Tier I Capital (to Risk-Weighted Assets), Actual Amount	$ 822,626	$ 772,558
Tier I Capital (to Risk-Weighted Assets), Actual Ratio	0.128	0.165
Tier I Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Amount	$ 384,678	$ 280,988
Tier I Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Ratio	0.06	0.06
Tier I Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Amount	$ 512,904	$ 374,650
Tier I Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Ratio	0.08	0.08
Common Equity Tier I Capital (to Risk-Weighted Assets)
Common Equity Tier I Capital (to Risk-Weighted Assets), Actual Amount	$ 822,626	$ 772,558
Common Equity Tier I Capital (to Risk-Weighted Assets), Actual Ratio	0.128	0.165
Common Equity Tier I Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Amount	$ 288,508	$ 210,741
Common Equity Tier I Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Ratio	0.045	0.045
Common Equity Tier I Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Amount	$ 416,734	$ 304,403
Common Equity Tier I Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Ratio	0.065	0.065
Tier I Capital (to Total Average Assets)
Tier I Capital (to Total Average Assets), Actual Amount	$ 822,626	$ 772,558
Tier I Capital (to Total Average Assets), Actual Ratio	0.133	0.153
Tier I Capital (to Total Average Assets), For minimum capital adequacy purposes, Amount	$ 247,709	$ 202,436
Tier I Capital (to Total Average Assets), For minimum capital adequacy purposes, Ratio	0.04	0.04
Tier I Capital (to Total Average Assets), To be well capitalized under prompt corrective action provisions, Amount	$ 309,636	$ 253,045
Tier I Capital (to Total Average Assets), To be well capitalized under prompt corrective action provisions, Ratio	0.05	0.05
Needham Bank [Member]
Total Capital (to Risk-Weighted Assets)
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 791,298	$ 656,103
Total Capital (to Risk-Weighted Assets), Actual Ratio	0.124	0.141
Total Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Amount	$ 511,589	$ 372,083
Total Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Ratio	0.08	0.08
Total Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Amount	$ 639,486	$ 465,104
Total Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Ratio	0.10	0.10
Tier I Capital (to Risk-Weighted Assets)
Tier I Capital (to Risk-Weighted Assets), Actual Amount	$ 742,881	$ 614,156
Tier I Capital (to Risk-Weighted Assets), Actual Ratio	0.116	0.132
Tier I Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Amount	$ 383,692	$ 279,063
Tier I Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Ratio	0.06	0.06
Tier I Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Amount	$ 511,589	$ 372,083
Tier I Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Ratio	0.08	0.08
Common Equity Tier I Capital (to Risk-Weighted Assets)
Common Equity Tier I Capital (to Risk-Weighted Assets), Actual Amount	$ 742,881	$ 614,156
Common Equity Tier I Capital (to Risk-Weighted Assets), Actual Ratio	0.116	0.132
Common Equity Tier I Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Amount	$ 287,769	$ 209,297
Common Equity Tier I Capital (to Risk-Weighted Assets), For minimum capital adequacy purposes, Ratio	0.045	0.045
Common Equity Tier I Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Amount	$ 415,666	$ 302,318
Common Equity Tier I Capital (to Risk-Weighted Assets), To be well capitalized under prompt corrective action provisions, Ratio	0.065	0.065
Tier I Capital (to Total Average Assets)
Tier I Capital (to Total Average Assets), Actual Amount	$ 742,881	$ 614,156
Tier I Capital (to Total Average Assets), Actual Ratio	0.122	0.125
Tier I Capital (to Total Average Assets), For minimum capital adequacy purposes, Amount	$ 244,395	$ 196,435
Tier I Capital (to Total Average Assets), For minimum capital adequacy purposes, Ratio	0.04	0.04
Tier I Capital (to Total Average Assets), To be well capitalized under prompt corrective action provisions, Amount	$ 305,494	$ 245,544
Tier I Capital (to Total Average Assets), To be well capitalized under prompt corrective action provisions, Ratio	0.05	0.05