Securities, HTM Securities in Continuous Unrealized Loss Position (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2025 USD ($) Position	Dec. 31, 2024 USD ($) Position
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 15,152	$ 16,605
12 months or longer	573,189	640,538
Total	588,341	657,143
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(316)	(114)
12 months or longer	(60,055)	(92,904)
Total	$ (60,371)	$ (93,018)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months \| Position	8	13
12 months or longer \| Position	230	276
Total \| Position	238	289
HTM debt securities, nonaccrual	$ 0	$ 0
Accrued interest reversed against interest income	0	0
Collateral-dependent HTM debt securities	0	0
Allowance for credit losses on HTM securities	0	0
AIR on HTM debt securities	$ 3,900	$ 4,400
Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Other Assets	Other Assets
Past Due [Member]
Unrealized Loss Position, Number of Positions [Abstract]
HTM debt securities	$ 0	$ 0
Federal Agency [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	0	0
12 months or longer	89,295	83,344
Total	89,295	83,344
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	0	0
12 months or longer	(10,705)	(16,656)
Total	$ (10,705)	$ (16,656)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months \| Position	0	0
12 months or longer \| Position	4	4
Total \| Position	4	4
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 10,771	$ 0
12 months or longer	168,422	189,271
Total	179,193	189,271
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(71)	0
12 months or longer	(23,308)	(34,865)
Total	$ (23,379)	$ (34,865)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months \| Position	1	0
12 months or longer \| Position	33	34
Total \| Position	34	34
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 0	$ 7,147
12 months or longer	173,333	198,978
Total	173,333	206,125
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	0	(7)
12 months or longer	(14,689)	(22,792)
Total	$ (14,689)	$ (22,799)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months \| Position	0	1
12 months or longer \| Position	47	52
Total \| Position	47	53
State & Municipal [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	$ 4,381	$ 9,458
12 months or longer	142,139	168,945
Total	146,520	178,403
Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	(245)	(107)
12 months or longer	(11,353)	(18,591)
Total	$ (11,598)	$ (18,698)
Unrealized Loss Position, Number of Positions [Abstract]
Less than 12 months \| Position	7	12
12 months or longer \| Position	146	186
Total \| Position	153	198
US Government Agencies Debt Securities and US Government-sponsored Enterprises Debt Securities [Member]
Unrealized Loss Position, Number of Positions [Abstract]
Held-to-maturity debt securities, percentage	65.00%	66.00%
Local Municipal Bonds [Member]
Unrealized Loss Position, Number of Positions [Abstract]
HTM debt securities	$ 85,700	$ 84,700

NBT BANCORP INC 10-K