Parent Company Financial Information, Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets [Abstract]
Cash and cash equivalents	$ 216,843	$ 180,955	$ 159,664	$ 149,181
Equity securities, at estimated fair value	27,771	23,053
Other assets	202,245	148,067
Total assets	9,715,925	9,556,363
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	8,595,528	8,538,454
Stockholders' equity	1,120,397	1,017,909	958,177	913,316
Total liabilities and stockholders' equity	9,715,925	9,556,363
NBT Bancorp Inc [Member]
Assets [Abstract]
Cash and cash equivalents	9,387	5,876	$ 7,572	$ 4,152
Equity securities, at estimated fair value	22,441	18,221
Investment in subsidiaries, on equity basis	1,209,752	1,117,350
Other assets	30,855	34,237
Total assets	1,272,435	1,175,684
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	152,038	157,775
Stockholders' equity	1,120,397	1,017,909
Total liabilities and stockholders' equity	$ 1,272,435	$ 1,175,684