Parent Company Financial Information, Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets [Abstract]
Cash and cash equivalents	$ 180,955	$ 159,664	$ 149,181	$ 140,297
Equity securities, at estimated fair value	23,053	0
Securities available for sale, at fair value	998,496	1,255,925
Trading securities	0	11,467
Other assets	148,067	128,548
Total assets	9,556,363	9,136,812
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	8,538,454	8,178,635
Stockholders' equity	1,017,909	958,177	913,316	882,004
Total liabilities and stockholders' equity	9,556,363	9,136,812
NBT Bancorp Inc [Member]
Assets [Abstract]
Cash and cash equivalents	5,876	7,572	$ 4,152	$ 28,682
Equity securities, at estimated fair value	18,221	0
Securities available for sale, at fair value	0	10,065
Trading securities	0	11,245
Investment in subsidiaries, on equity basis	1,117,350	1,048,908
Other assets	34,237	40,461
Total assets	1,175,684	1,118,251
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	157,775	160,074
Stockholders' equity	1,017,909	958,177
Total liabilities and stockholders' equity	$ 1,175,684	$ 1,118,251