Parent Company Financial Information, Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Consolidated Statements of Income [Abstract]
Net securities (losses) gains	$ (1,131)	$ 566	$ (388)
Interest, dividends and other income	384,070	339,876	348,282
Income before income tax expense	196,156	199,858	133,087
Income tax benefit	44,161	44,973	28,699
Net income	151,995	154,885	104,388
NBT Bancorp Inc [Member]
Consolidated Statements of Income [Abstract]
Dividends from subsidiaries	119,000	118,900	77,000
Management fee from subsidiaries	2,005	2,653	4,151
Net securities (losses) gains	(618)	543	(483)
Interest, dividends and other income	638	564	824
Total revenue	121,025	122,660	81,492
Operating expenses	14,035	11,956	11,825
Income before income tax expense	106,990	110,704	69,667
Income tax benefit	(3,027)	(2,250)	(2,653)
Equity in undistributed income of subsidiaries	41,978	41,931	32,068
Net income	$ 151,995	$ 154,885	$ 104,388