Parent Company Financial Information, Condensed Income Statements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Income Statements [Abstract]
Net securities gains	$ 1,867	$ (644)	$ 3,087
Interest, dividends and other income	309,407	286,947	273,224
Income before income tax benefit and equity in undistributed income of subsidiaries	128,161	118,801	116,628
Income tax expense (benefit)	46,010	40,392	40,203
Net income	82,151	78,409	76,425
NBT Bancorp Inc [Member]
Condensed Income Statements [Abstract]
Dividends from subsidiaries	38,300	10,200	78,200
Management fee from subsidiaries	99,319	95,244	92,629
Net securities gains	2,237	652	3,034
Interest, dividends and other income	928	976	693
Total revenue	140,784	107,072	174,556
Operating expenses	100,667	97,977	94,332
Income before income tax benefit and equity in undistributed income of subsidiaries	40,117	9,095	80,224
Income tax expense (benefit)	2,233	(321)	515
Dividends in excess of income (equity in undistributed income) of subsidiaries	44,267	68,993	(3,284)
Net income	$ 82,151	$ 78,409	$ 76,425