Parent Company Financial Information, Condensed Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities [Abstract]
Net income	$ 121,021	$ 112,566	$ 82,151
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	9,497	9,280	9,056
Excess tax benefit on stock-based compensation	(409)	(543)	(1,769)
Stock-based compensation expense	4,210	3,936	3,644
Net security (gains) losses	(4,213)	6,341	(1,867)
Re-evaluation of deferred tax amounts from Tax Act	0	0	4,407
Bank owned life insurance income	(5,355)	(5,091)	(5,175)
Net change in other assets and other liabilities	(4,902)	(9,554)	7,400
Net cash provided by operating activities	153,463	147,773	136,904
Investing activities [Abstract]
Proceeds on sales of equity securities	3,966	3,318	0
Purchases of equity securities	(93)	(2)	0
Proceeds on sales and maturities of AFS securities	273,578	259,446	290,613
Purchases of AFS securities	(252,963)	(132,448)	(233,804)
Proceeds from settlement of bank owned life insurance	1,086	0	799
Net purchases of premises and equipment	(6,647)	(7,402)	(6,691)
Net cash (used in) provided by investing activities	(64,343)	(417,619)	(305,193)
Financing activities [Abstract]
Proceeds from the issuance of shares to employee and other stock plans	725	1,296	3,309
Cash paid by employer for tax-withholdings on stock issuance	(1,622)	(1,893)	(3,582)
Cash dividends	(46,010)	(43,269)	(40,104)
Net cash (used in) provided by financing activities	(53,232)	291,137	178,772
Net increase (decrease) in cash and cash equivalents	35,888	21,291	10,483
Cash and cash equivalents at beginning of year	180,955	159,664	149,181
Cash and cash equivalents at end of year	216,843	180,955	159,664
NBT Bancorp Inc [Member]
Operating activities [Abstract]
Net income	121,021	112,566	82,151
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	2,298	2,814	2,974
Excess tax benefit on stock-based compensation	(409)	(543)	(1,769)
Stock-based compensation expense	4,210	3,936	3,644
Net security (gains) losses	(165)	(399)	(2,238)
Re-evaluation of deferred tax amounts from Tax Act	0	0	3,339
Equity in undistributed income of subsidiaries	(124,673)	(115,972)	(82,567)
Cash dividend from subsidiaries	50,200	43,000	38,300
Bank owned life insurance income	(398)	(424)	(328)
Net change in other assets and other liabilities	(1,573)	(6,124)	(2,454)
Net cash provided by operating activities	50,511	38,854	41,052
Investing activities [Abstract]
Proceeds on sales of equity securities	0	3,318	0
Purchases of equity securities	(93)	(2)	0
Proceeds on sales and maturities of AFS securities	0	0	4,710
Purchases of AFS securities	0	0	(9)
Proceeds from settlement of bank owned life insurance	0	0	308
Net purchases of premises and equipment	0	0	(2,264)
Net cash (used in) provided by investing activities	(93)	3,316	2,745
Financing activities [Abstract]
Proceeds from the issuance of shares to employee and other stock plans	725	1,296	3,309
Cash paid by employer for tax-withholdings on stock issuance	(1,622)	(1,893)	(3,582)
Cash dividends	(46,010)	(43,269)	(40,104)
Net cash (used in) provided by financing activities	(46,907)	(43,866)	(40,377)
Net increase (decrease) in cash and cash equivalents	3,511	(1,696)	3,420
Cash and cash equivalents at beginning of year	5,876	7,572	4,152
Cash and cash equivalents at end of year	$ 9,387	$ 5,876	$ 7,572