Parent Company Financial Information, Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating activities [Abstract]
Net income	$ 154,885	$ 104,388	$ 121,021
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	9,896	9,772	9,497
Excess tax benefit on stock-based compensation	(385)	(181)	(409)
Stock-based compensation expense	4,414	4,581	4,210
Net securities (gains) losses	(566)	388	(4,213)
Bank owned life insurance income	(6,217)	(5,743)	(5,355)
Amortization of subordinated debt issuance costs	438	230	0
Net change in other assets and other liabilities	(12,667)	(49,930)	(6,774)
Net cash provided by operating activities	157,635	142,412	153,463
Investing activities [Abstract]
Purchases of equity securities	0	0	(93)
Net cash provided by (used in) investing activities	(546,058)	(709,725)	(64,343)
Financing activities [Abstract]
Proceeds from issuance of subordinated debt	0	100,000	0
Payment of subordinated debt issuance costs	0	(2,178)	0
Proceeds from the issuance of shares to employee and other stock plans	112	184	725
Cash paid by employer for tax-withholding on stock issuance	(2,931)	(1,537)	(1,622)
Purchases of treasury shares	(21,714)	(7,980)	0
Cash dividends	(47,738)	(47,207)	(46,010)
Net cash provided by (used in) financing activities	984,813	1,023,151	(53,232)
Net increase in cash and cash equivalents	596,390	455,838	35,888
NBT Bancorp Inc [Member]
Operating activities [Abstract]
Net income	154,885	104,388	121,021
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	1,113	1,739	2,298
Excess tax benefit on stock-based compensation	(385)	(181)	(409)
Stock-based compensation expense	4,414	4,581	4,210
Net securities (gains) losses	(543)	483	(165)
Equity in undistributed income of subsidiaries	(41,931)	(32,068)	(74,473)
Bank owned life insurance income	(326)	(391)	(398)
Amortization of subordinated debt issuance costs	438	230	0
Net change in other assets and other liabilities	(7,127)	(3,535)	(1,573)
Net cash provided by operating activities	110,538	75,246	50,511
Investing activities [Abstract]
Investment in the Bank	0	(90,000)	0
Proceeds from calls of equity securities	1,000	2,000	0
Purchases of equity securities	0	0	(93)
Net cash provided by (used in) investing activities	1,000	(88,000)	(93)
Financing activities [Abstract]
Proceeds from issuance of subordinated debt	0	100,000	0
Payment of subordinated debt issuance costs	0	(2,178)	0
Proceeds from the issuance of shares to employee and other stock plans	112	184	725
Cash paid by employer for tax-withholding on stock issuance	(2,931)	(1,537)	(1,622)
Purchases of treasury shares	(21,714)	(7,980)	0
Cash dividends	(47,738)	(47,207)	(46,010)
Net cash provided by (used in) financing activities	(72,271)	41,282	(46,907)
Net increase in cash and cash equivalents	39,267	28,528	3,511
Cash and cash equivalents at beginning of year	37,915	9,387	5,876
Cash and cash equivalents at end of year	$ 77,182	$ 37,915	$ 9,387