Parent Company Financial Information, Condensed Income Statements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Income Statements [Abstract]
Net securities gains	$ 4,213	$ (6,341)	$ 1,867
Interest, dividends and other income	367,534	344,255	309,407
Income before income tax benefit and equity in undistributed income of subsidiaries	155,432	137,002	128,161
Income tax expense (benefit)	34,411	24,436	46,010
Net income	121,021	112,566	82,151
NBT Bancorp Inc [Member]
Condensed Income Statements [Abstract]
Dividends from subsidiaries	50,200	43,000	38,300
Management fee from subsidiaries	7,981	7,907	99,319
Net securities gains	165	399	2,237
Interest, dividends and other income	876	905	928
Total revenue	59,222	52,211	140,784
Operating expenses	14,262	14,226	100,667
Income before income tax benefit and equity in undistributed income of subsidiaries	44,960	37,985	40,117
Income tax expense (benefit)	(1,588)	(1,608)	2,233
Dividends in excess of income (equity in undistributed income) of subsidiaries	74,473	72,973	44,267
Net income	$ 121,021	$ 112,566	$ 82,151