Loans Receivable and Allowance for Credit Losses - Narrative (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2026 USD ($) property loan	Mar. 31, 2025 USD ($)	Dec. 31, 2025 USD ($) real_estate loan	Dec. 31, 2024 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale	$ 0		$ 0
Gain on sale of loans held-for-sale	20,000	$ 515,000
Loss on sale of loans held-for-sale	0	0
Purchases of loans receivable	0	20,000,000
Carrying value of servicing liability	$ 485,900,000		494,800,000
Threshold period, past due status of financial receivables	30 days
Non-accrual	$ 41,400,000		$ 38,000,000.0
Threshold period, past due for nonperforming status of financial receivables	90 days		90 days
Financing receivable, excluding accrued interest, 90 days or more past due, still accruing	$ 0		$ 0
Loans receivable	8,259,678,000		$ 8,292,010,000
Charge-offs	$ 79,000	991,000
Number of real estate acquired through foreclosure \| property	1
Real estate acquired through foreclosure	$ 5,900,000
Number of real estate owned \| real_estate			0
Accrued interest receivable on loans receivable	34,700,000		$ 34,700,000
Off-balance sheet, credit loss, liability	4,212,000	4,289,000	3,946,000	$ 3,821,000
Provision for credit losses	266,000	468,000
Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs			3,200,000
Stewardship Financial Corporation \| Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,100,000		1,100,000
Freehold Entities \| Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	40,000		44,000
RSI Entites \| Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	8,200,000		8,300,000
Equipment Finance Loans \| Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	850,000		1,000,000.0
Less Than 90 Days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	$ 13,800,000		$ 10,400,000
Financial receivable, number of loans in nonaccrual status \| loan	41		38
Real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales of loans held-for-sale	$ 252,000	2,000,000.0
Real estate loans \| One-to-four family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales of loans held-for-sale		5,200,000
Charge-offs	$ 0	0
Mortgage loans in process of foreclosure, number of loans \| loan	8
Mortgage loans in process of foreclosure, amount	$ 2,400,000
Real estate loans \| Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales of loans held-for-sale	2,000,000.0
Purchases of loans receivable	0	20,000,000.0
Charge-offs	0	53,000
Commercial business loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales of loans held-for-sale		5,500,000
Home equity loans and advances \| Home equity loans and advances
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	$ 0	$ 0
Mortgage loans in process of foreclosure, number of loans \| loan	4		4
Mortgage loans in process of foreclosure, amount	$ 228,000		$ 585,000
Home equity loans and advances \| Residential Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage loans in process of foreclosure, number of loans \| loan			9
Mortgage loans in process of foreclosure, amount			$ 2,500,000

Columbia Financial, Inc. 10-Q/A