Parent Company Financial Information, Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets [Abstract]
Cash and cash equivalents	$ 672,681	$ 216,843	$ 180,955	$ 159,664
Equity securities, at estimated fair value	30,737	27,771
Other assets	293,957	202,245
Total assets	10,932,906	9,715,925
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	9,745,288	8,595,528
Stockholders' equity	1,187,618	1,120,397	1,017,909	958,177
Total liabilities and stockholders' equity	10,932,906	9,715,925
NBT Bancorp Inc [Member]
Assets [Abstract]
Cash and cash equivalents	37,915	9,387	$ 5,876	$ 7,572
Equity securities, at estimated fair value	25,111	22,441
Investment in subsidiaries, on equity basis	1,345,317	1,209,752
Other assets	30,577	30,855
Total assets	1,438,920	1,272,435
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	251,302	152,038
Stockholders' equity	1,187,618	1,120,397
Total liabilities and stockholders' equity	$ 1,438,920	$ 1,272,435