LOANS, ALLOWANCE FOR LOAN LOSSES AND CREDIT QUALITY (Details) - USD ($)			12 Months Ended
		Jul. 01, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Receivables [Abstract]
Days To Be Termed As Non Accrual Loans			90 days
Financing Receivable, Allowance for Credit Loss [Line Items]
Increase in allowance for credit loss			$ (19,900,000)
Allowance for credit loss			(189,877,000)	$ (169,984,000)
Financing Receivable, before Allowance for Credit Loss	[1]		18,503,777,000	14,508,378,000
Provision for credit losses			65,469,000	36,250,000	$ 23,250,000
Accounts, Notes, Loans and Financing Receivable [Line Items]
Off-Balance Sheet, Credit Loss, Liability			1,800,000	1,400,000
Financing Receivable, Unamortized Loan Cost (Fee)			7,700,000	6,100,000
Interest Receivable			70,300,000	55,600,000	60,200,000
Accounts Receivable, Noncurrent, Accrued Interest, Writeoff			1,000,000.0	$ 676,000	$ 1,000,000.0
Enterprise Bancorp, Inc. [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Initial allowance reserves			43,500,000
Provision for credit losses		$ 34,500,000
Enterprise Bancorp, Inc. - Non-PCD Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Initial allowance reserves			34,500,000
Enterprise Bancorp, Inc. PCD Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Initial allowance reserves			$ 9,000,000.0

[1] (2)Loan origination dates in the tables above reflect the original date, or the date of a material modification of a previously originated loan, for both organic originations and acquired loans. (3)Other consumer portfolio is inclusive of deposit account overdrafts recorded as loan balances and the associated gross write-offs.