Parent Company Financial Information, Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets [Abstract]
Cash and cash equivalents	$ 149,181	$ 140,297	$ 146,636	$ 158,926
Securities available for sale, at estimated fair value	1,338,290	1,174,544
Trading securities	9,259	8,377
Other assets	129,247	122,517
Total assets	8,867,268	8,262,646
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	7,953,952	7,380,642
Stockholders' equity	913,316	882,004	864,181	816,569
Total liabilities and stockholders' equity	8,867,268	8,262,646
NBT Bancorp Inc [Member]
Assets [Abstract]
Cash and cash equivalents	4,152	28,682	$ 3,586	$ 3,741
Securities available for sale, at estimated fair value	15,273	12,711
Trading securities	8,968	8,042
Investment in subsidiaries, on equity basis	1,006,444	941,731
Other assets	44,178	65,133
Total assets	1,079,015	1,056,299
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	165,699	174,295
Stockholders' equity	913,316	882,004
Total liabilities and stockholders' equity	$ 1,079,015	$ 1,056,299