Parent Company Financial Information, Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Operating activities [Abstract]
Net income	$ 118,782	$ 151,995	$ 154,885
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	10,695	10,155	9,896
Excess tax benefit on stock-based compensation	(296)	(288)	(385)
Stock-based compensation expense	5,102	4,530	4,414
Net securities losses (gains)	9,315	1,131	(566)
Bank owned life insurance income	(6,750)	(6,044)	(6,217)
Amortization of subordinated debt issuance costs	437	437	438
Discount on repurchase of subordinated debt	0	(106)	0
Net change in other assets and other liabilities	(27,907)	11,932	(11,981)
Net cash provided by operating activities	157,457	183,223	159,185
Investing activities [Abstract]
Net cash provided by (used in) acquisitions	44,564	(2,616)	(1,550)
Net cash (used in) provided by investing activities	(44,226)	(926,216)	(547,608)
Financing activities [Abstract]
Repurchase of subordinated debt	0	(2,000)	0
Proceeds from the issuance of shares to employee and other stock plans	91	0	112
Cash paid by employer for tax-withholding on stock issuance	(1,877)	(1,751)	(2,931)
Purchases of treasury shares	(4,944)	(14,713)	(21,714)
Cash dividends	(55,886)	(49,765)	(47,738)
Net cash (used in) provided by financing activities	(105,392)	(328,728)	984,813
Net increase (decrease) in cash and cash equivalents	7,839	(1,071,721)	596,390
NBT Bancorp Inc [Member]
Operating activities [Abstract]
Net income	118,782	151,995	154,885
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	353	582	1,113
Excess tax benefit on stock-based compensation	(296)	(288)	(385)
Stock-based compensation expense	5,102	4,530	4,414
Net securities losses (gains)	82	618	(543)
Equity in undistributed income of subsidiaries	(13,950)	(41,978)	(41,931)
Bank owned life insurance income	(271)	(238)	(326)
Amortization of subordinated debt issuance costs	437	437	438
Discount on repurchase of subordinated debt	0	(106)	0
Net change in other assets and other liabilities	(4,930)	(8,376)	(7,127)
Net cash provided by operating activities	105,309	107,176	110,538
Investing activities [Abstract]
Net cash provided by (used in) acquisitions	3,542	0	0
Proceeds from calls of equity securities	0	0	1,000
Net cash (used in) provided by investing activities	3,542	0	1,000
Financing activities [Abstract]
Repurchase of subordinated debt	0	(2,000)	0
Proceeds from the issuance of shares to employee and other stock plans	91	0	112
Cash paid by employer for tax-withholding on stock issuance	(1,877)	(1,751)	(2,931)
Purchases of treasury shares	(4,944)	(14,713)	(21,714)
Cash dividends	(55,886)	(49,765)	(47,738)
Net cash (used in) provided by financing activities	(62,616)	(68,229)	(72,271)
Net increase (decrease) in cash and cash equivalents	46,235	38,947	39,267
Cash and cash equivalents at beginning of year	116,129	77,182	37,915
Cash and cash equivalents at end of year	$ 162,364	$ 116,129	$ 77,182