Parent Company Financial Information, Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets [Abstract]
Cash and cash equivalents	$ 159,664	$ 149,181	$ 140,297	$ 146,636
Securities available for sale, at fair value	1,255,925	1,338,290
Trading securities	11,467	9,259
Other assets	128,548	129,247
Total assets	9,136,812	8,867,268
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	8,178,635	7,953,952
Stockholders' equity	958,177	913,316	882,004	864,181
Total liabilities and stockholders' equity	9,136,812	8,867,268
NBT Bancorp Inc [Member]
Assets [Abstract]
Cash and cash equivalents	7,572	4,152	$ 28,682	$ 3,586
Securities available for sale, at fair value	10,065	15,273
Trading securities	11,245	8,968
Investment in subsidiaries, on equity basis	1,048,908	1,006,444
Other assets	40,461	44,178
Total assets	1,118,251	1,079,015
Liabilities and Stockholders' Equity [Abstract]
Total liabilities	160,074	165,699
Stockholders' equity	958,177	913,316
Total liabilities and stockholders' equity	$ 1,118,251	$ 1,079,015