Organization, Activities and Significant Accounting Policies - Conversion (Details)		12 Months Ended
	Dec. 27, 2023 USD ($) $ / shares shares	Dec. 31, 2024 USD ($) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2025 shares
Organization, Activities and Significant Accounting Policies
Issuance of common shares donated to Needham Bank Charitable Foundation (in shares)	1,708,229
Common Stock, shares issued		42,705,729		45,770,128
Common stock, shares outstanding		42,705,729		45,770,128
Number of shares under ESOP		2,416,458	1,000,000
Conversion costs | $		$ 225,000	$ 9,600,000
Public offering
Organization, Activities and Significant Accounting Policies
Percentage of transfer of common stock upon reorganization	100
Number of shares sold pursuant to the plan	40,997,500
Share price | $ / shares	$ 10
Net proceeds received upon conversion | $	$ 400,400,000
Donation in cash to Needham Bank Charitable Foundation | $	$ 2,000,000