Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	$ 1,122,017	$ 1,025,946
Equity securities	6,802	6,673
U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	398,470	314,702
Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	654,973	622,957
Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	1,961	2,359
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	66,613	85,928
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	398,470	314,702
Equity securities	6,471	6,350
Derivative assets	0	0
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	711,909	700,743
Equity securities	331	323
Derivative assets	10,525	18,895
Derivative liabilities	13,503	20,025
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	11,638	10,501
Equity securities		0
Derivative assets	0	0
Derivative liabilities	0	0
Measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	1,122,017	1,025,946
Equity securities	6,802	6,673
Derivative assets	10,525	18,895
Assets	1,139,344	1,051,514
Derivative liabilities	13,503	20,025
Measured on recurring basis \| U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	398,470	314,702
Measured on recurring basis \| Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	654,973	622,957
Measured on recurring basis \| Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	1,961	2,359
Measured on recurring basis \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	66,613	85,928
Measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	398,470	314,702
Equity securities	6,471	6,350
Derivative assets	0	0
Assets	404,941	321,052
Derivative liabilities	0	0
Measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	398,470	314,702
Measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	0	0
Measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	0	0
Measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	0	0
Measured on recurring basis \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	711,909	700,743
Equity securities	331	323
Derivative assets	10,525	18,895
Assets	722,765	719,961
Derivative liabilities	13,503	20,025
Measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	0	0
Measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	654,973	622,957
Measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	425	426
Measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	56,511	77,360
Measured on recurring basis \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	11,638	10,501
Equity securities	0	0
Derivative assets	0	0
Assets	11,638	10,501
Derivative liabilities	0	0
Measured on recurring basis \| Significant Unobservable Inputs (Level 3) \| U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	0	0
Measured on recurring basis \| Significant Unobservable Inputs (Level 3) \| Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	0	0
Measured on recurring basis \| Significant Unobservable Inputs (Level 3) \| Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	1,536	1,933
Measured on recurring basis \| Significant Unobservable Inputs (Level 3) \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale, at fair value	$ 10,102	$ 8,568

Columbia Financial, Inc. 10-K