Parent Company Financial Information, Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Assets [Abstract]
Equity securities, at estimated fair value	$ 30,784	$ 33,550
Other assets	379,797	266,733
Total assets	11,739,296	12,012,111
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	10,565,742	10,761,658
Stockholders' equity	1,173,554	1,250,453	$ 1,187,618	$ 1,120,397
Total liabilities and stockholders' equity	11,739,296	12,012,111
NBT Bancorp Inc [Member]
Assets [Abstract]
Cash and cash equivalents	116,129	77,182	$ 37,915	$ 9,387
Equity securities, at estimated fair value	24,499	27,622
Investment in subsidiaries, on equity basis	1,245,459	1,359,613
Other assets	39,339	36,216
Total assets	1,425,426	1,500,633
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	251,872	250,180
Stockholders' equity	1,173,554	1,250,453
Total liabilities and stockholders' equity	$ 1,425,426	$ 1,500,633