v3.25.3

Debt Securities Held to Maturity - Narrative (Details)	3 Months Ended		9 Months Ended
Debt Securities Held to Maturity - Narrative (Details)	Sep. 30, 2025 USD ($) security	Sep. 30, 2024 USD ($) security	Sep. 30, 2025 USD ($) security	Sep. 30, 2024 USD ($) security	Dec. 31, 2024 USD ($) security
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 399,278,000		$ 399,278,000		$ 392,840,000
Debt securities held to maturity	368,206,000		368,206,000		350,153,000
Proceeds from sale and maturity of held-to-maturity securities	$ 0	$ 0	$ 0	$ 0
Debt securities, held-to-maturity, number of called debt securities \| security	0	1	0	1
Proceeds from maturities, calls of held-to-maturity securities		$ 5,000,000.0		$ 5,000,000.0
Fair value of securities sold under agreements to repurchase	$ 229,800,000		$ 229,800,000		$ 247,600,000
Number of positions \| security	102		102		105
Accrued interest receivable on debt securities held-to-maturity	$ 597,000		$ 597,000		$ 898,000
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	354,406,000		354,406,000		347,969,000
Debt securities held to maturity	$ 326,963,000		$ 326,963,000		$ 310,570,000

Debt Securities, Held-to-Maturity, Number Of Called Debt Securities

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Proceeds From Maturities, Calls Of Held-to-Maturity Securities

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Amount, after allowance for credit loss, of accrued interest on investment in debt security measured at amortized cost (held-to-maturity).

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Amount excluding accrued interest, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Number of investment positions in unrealized loss position for held-to-maturity security where cost exceeds fair value and its impairment is not other-than-temporary.

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Amount of cash inflow from the sale or maturity of long-term held-to-maturity securities.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Columbia Financial, Inc. 10-Q