Securities - Additional Information (Details) - USD ($)	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, allowance for credit loss, excluding accrued interest	$ 0	$ 0	$ 0
Debt securities, available-for-sale, accrued interest, after allowance for credit loss	12,400,000		11,900,000
Debt securities, available-for-sale, accrued interest writeoff	0	0
Debt securities, held-to-maturity, allowance for credit loss, excluding accrued interest	0	0	0
Debt securities, held-to-maturity, accrued interest, after allowance for credit loss	4,900,000		3,600,000
Debt securities, held-to-maturity, accrued interest, writeoff	0	$ 0
Federal Home Loan Bank of Boston
Debt Securities, Available-for-sale [Line Items]
Deposit liabilities, collateral issued, financial instruments	1,500,000,000		200,000,000
Other Purposes Required By Law
Debt Securities, Available-for-sale [Line Items]
Deposit liabilities, collateral issued, financial instruments	686,400,000		692,900,000
Federal Reserve Discount Window
Debt Securities, Available-for-sale [Line Items]
Deposit liabilities, collateral issued, financial instruments	$ 397,100,000		$ 414,200,000

Amount, after allowance for credit loss, of accrued interest on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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