Parent Company Financial Information, Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Assets [Abstract]
Equity securities, at estimated fair value	$ 37,591	$ 30,784
Other assets	395,889	379,797
Total assets	13,309,040	11,739,296
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	11,883,349	10,565,742
Stockholders' equity	1,425,691	1,173,554	$ 1,250,453	$ 1,187,618
Total liabilities and stockholders' equity	13,309,040	11,739,296
NBT Bancorp Inc [Member]
Assets [Abstract]
Cash and cash equivalents	162,364	116,129	$ 77,182	$ 37,915
Equity securities, at estimated fair value	28,739	24,499
Investment in subsidiaries, on equity basis	1,464,980	1,245,459
Other assets	42,435	39,339
Total assets	1,698,518	1,425,426
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	272,827	251,872
Stockholders' equity	1,425,691	1,173,554
Total liabilities and stockholders' equity	$ 1,698,518	$ 1,425,426