Parent Company Financial Information, Condensed Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities [Abstract]
Net income	$ 112,566	$ 82,151	$ 78,409
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	9,280	9,056	9,023
Excess tax (benefit) on stock-based compensation	(543)	(1,769)	(1,055)
Stock-based compensation expense	3,936	3,644	4,378
Net securities gain	6,622	(1,869)	644
Re-evaluation of deferred tax amounts from Tax Act	0	4,407	0
Bank owned life insurance income	(5,091)	(5,175)	(5,195)
Net change in other assets and other liabilities	(9,554)	7,400	(8,223)
Net cash provided by operating activities	147,773	136,904	110,565
Investing activities [Abstract]
Proceeds on sales of equity securities	3,318	0	0
Purchases of equity securities	(2)	0	0
Proceeds on sales and maturities of AFS securities	259,446	290,613	324,781
Purchases of AFS securities	(132,448)	(233,804)	(597,428)
Proceeds from settlement of bank owned life insurance	0	799	1,477
Net purchases of premises and equipment	(7,402)	(6,691)	(3,308)
Net cash provided by (used in) investing activities	(417,619)	(305,193)	(629,960)
Financing activities [Abstract]
Proceeds from the issuance of shares to employee and other stock plans	1,296	3,309	6,032
Cash paid by employer for tax-withholdings on stock issuance	(1,893)	(3,582)	(3,387)
Purchases of treasury shares	0	0	(17,193)
Cash dividends	(43,269)	(40,104)	(38,880)
Net cash (used in) financing activities	291,137	178,772	528,279
Net (decrease) increase in cash and cash equivalents	21,291	10,483	8,884
Cash and cash equivalents at beginning of year	159,664	149,181	140,297
Cash and cash equivalents at end of year	180,955	159,664	149,181
NBT Bancorp Inc [Member]
Operating activities [Abstract]
Net income	112,566	82,151	78,409
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	2,814	2,974	2,805
Excess tax (benefit) on stock-based compensation	(543)	(1,769)	(1,055)
Stock-based compensation expense	3,936	3,644	4,378
Net securities gain	(399)	(2,238)	(652)
Re-evaluation of deferred tax amounts from Tax Act	0	3,339	0
Equity in undistributed income of subsidiaries	(115,972)	(82,567)	(79,193)
Cash dividend from subsidiaries	43,000	38,300	10,200
Bank owned life insurance income	(424)	(328)	(356)
Net change in other assets and other liabilities	(6,124)	(2,454)	16,242
Net cash provided by operating activities	38,854	41,052	30,778
Investing activities [Abstract]
Proceeds on sales of equity securities	3,318	0	0
Purchases of equity securities	(2)	0	0
Proceeds on sales and maturities of AFS securities	0	4,710	1,783
Purchases of AFS securities	0	(9)	(580)
Proceeds from settlement of bank owned life insurance	0	308	0
Net purchases of premises and equipment	0	(2,264)	(3,083)
Net cash provided by (used in) investing activities	3,316	2,745	(1,880)
Financing activities [Abstract]
Proceeds from the issuance of shares to employee and other stock plans	1,296	3,309	6,032
Cash paid by employer for tax-withholdings on stock issuance	(1,893)	(3,582)	(3,387)
Purchases of treasury shares	0	0	(17,193)
Cash dividends	(43,269)	(40,104)	(38,880)
Net cash (used in) financing activities	(43,866)	(40,377)	(53,428)
Net (decrease) increase in cash and cash equivalents	(1,696)	3,420	(24,530)
Cash and cash equivalents at beginning of year	7,572	4,152	28,682
Cash and cash equivalents at end of year	$ 5,876	$ 7,572	$ 4,152