Parent Company Financial Information, Condensed Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities [Abstract]
Net income	$ 104,388	$ 121,021	$ 112,566
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	9,772	9,497	9,280
Excess tax benefit on stock-based compensation	(181)	(409)	(543)
Stock-based compensation expense	4,581	4,210	3,936
Net securities losses (gains)	388	(4,213)	6,341
Bank owned life insurance income	(5,743)	(5,355)	(5,091)
Amortization of subordinated debt issuance costs	230	0	0
Net change in other assets and other liabilities	(49,930)	(6,774)	(9,554)
Net cash provided by operating activities	142,412	153,463	147,773
Investing activities [Abstract]
Proceeds on sales of equity securities	0	3,966	3,318
Purchases of equity securities	0	(93)	(2)
Net cash used in investing activities	(709,725)	(64,343)	(417,619)
Financing activities [Abstract]
Proceeds from issuance of subordinated debt	100,000	0	0
Payment of subordinated debt issuance costs	(2,178)	0	0
Proceeds from the issuance of shares to employee and other stock plans	184	725	1,296
Cash paid by employer for tax-withholding on stock issuance	(1,537)	(1,622)	(1,893)
Purchases of treasury shares	(7,980)	0	0
Cash dividends	(47,207)	(46,010)	(43,269)
Net cash provided by (used in) financing activities	1,023,151	(53,232)	291,137
Net increase in cash and cash equivalents	455,838	35,888	21,291
Cash and cash equivalents at beginning of year	216,843	180,955	159,664
Cash and cash equivalents at end of year	672,681	216,843	180,955
NBT Bancorp Inc [Member]
Operating activities [Abstract]
Net income	104,388	121,021	112,566
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	1,739	2,298	2,814
Excess tax benefit on stock-based compensation	(181)	(409)	(543)
Stock-based compensation expense	4,581	4,210	3,936
Net securities losses (gains)	483	(165)	(399)
Equity in undistributed income of subsidiaries	(32,068)	(74,473)	(72,973)
Bank owned life insurance income	(391)	(398)	(424)
Amortization of subordinated debt issuance costs	230	0	0
Net change in other assets and other liabilities	(3,535)	(1,573)	(6,124)
Net cash provided by operating activities	75,246	50,511	38,853
Investing activities [Abstract]
Investment in the Bank	(90,000)	0	0
Proceeds from calls of equity securities	2,000	0	0
Proceeds on sales of equity securities	0	0	3,318
Purchases of equity securities	0	(93)	(2)
Net cash used in investing activities	(88,000)	(93)	3,316
Financing activities [Abstract]
Proceeds from issuance of subordinated debt	100,000	0	0
Payment of subordinated debt issuance costs	(2,178)	0	0
Proceeds from the issuance of shares to employee and other stock plans	184	725	1,296
Cash paid by employer for tax-withholding on stock issuance	(1,537)	(1,622)	(1,893)
Purchases of treasury shares	(7,980)	0	0
Cash dividends	(47,207)	(46,010)	(43,269)
Net cash provided by (used in) financing activities	41,282	(46,907)	(43,866)
Net increase in cash and cash equivalents	28,528	3,511	(1,697)
Cash and cash equivalents at beginning of year	9,387	5,876	7,572
Cash and cash equivalents at end of year	$ 37,915	$ 9,387	$ 5,876