Parent Company Financial Information, Condensed Income Statements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements [Abstract]
Net securities gains	$ (6,341)	$ 1,867	$ (644)
Interest, dividends and other income	344,255	309,407	286,947
Income before income tax benefit and equity in undistributed income of subsidiaries	137,002	128,161	118,801
Income tax (benefit) expense	24,436	46,010	40,392
Net income	112,566	82,151	78,409
NBT Bancorp Inc [Member]
Condensed Income Statements [Abstract]
Dividends from subsidiaries	43,000	38,300	10,200
Management fee from subsidiaries	7,907	99,319	95,244
Net securities gains	399	2,237	652
Interest, dividends and other income	905	928	976
Total revenue	52,211	140,784	107,072
Operating expenses	14,226	100,667	97,977
Income before income tax benefit and equity in undistributed income of subsidiaries	37,985	40,117	9,095
Income tax (benefit) expense	(1,608)	2,233	(321)
Dividends in excess of income (equity in undistributed income) of subsidiaries	72,973	44,267	68,993
Net income	$ 112,566	$ 82,151	$ 78,409