Parent Company Financial Information, Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Consolidated Statements of Income [Abstract]
Net securities (losses) gains	$ (9,315)	$ (1,131)	$ 566
Interest, dividends and other income	522,789	384,070	339,876
Income before income tax expense	153,459	196,156	199,858
Income tax expense (benefit)	34,677	44,161	44,973
Net income	118,782	151,995	154,885
NBT Bancorp Inc [Member]
Consolidated Statements of Income [Abstract]
Dividends from subsidiaries	116,250	119,000	118,900
Management fee from subsidiaries	7,093	2,005	2,653
Net securities (losses) gains	(82)	(618)	543
Interest, dividends and other income	715	638	564
Total revenue	123,976	121,025	122,660
Operating expenses	22,930	14,035	11,956
Income before income tax expense	101,046	106,990	110,704
Income tax expense (benefit)	(3,785)	(3,027)	(2,250)
Equity in undistributed income of subsidiaries	13,951	41,978	41,931
Net income	$ 118,782	$ 151,995	$ 154,885