Parent Company Financial Information, Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Consolidated Statements of Income [Abstract]
Net securities gains (losses)	$ 566	$ (388)	$ 4,213
Interest, dividends and other income	339,876	348,282	367,534
Income before income tax expense	199,858	133,087	155,432
Income tax benefit	44,973	28,699	34,411
Net income	154,885	104,388	121,021
NBT Bancorp Inc [Member]
Consolidated Statements of Income [Abstract]
Dividends from subsidiaries	118,900	77,000	50,200
Management fee from subsidiaries	2,653	4,151	7,981
Net securities gains (losses)	543	(483)	165
Interest, dividends and other income	564	824	876
Total revenue	122,660	81,492	59,222
Operating expenses	11,956	11,825	14,262
Income before income tax expense	110,704	69,667	44,960
Income tax benefit	(2,250)	(2,653)	(1,588)
Equity in undistributed income of subsidiaries	41,931	32,068	74,473
Net income	$ 154,885	$ 104,388	$ 121,021