Parent Company Financial Information, Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Operating activities [Abstract]
Net income	$ 151,995	$ 154,885	$ 104,388
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	10,155	9,896	9,772
Excess tax benefit on stock-based compensation	(288)	(385)	(181)
Stock-based compensation expense	4,530	4,414	4,581
Net security losses (gains)	1,131	(566)	388
Bank owned life insurance income	(6,044)	(6,217)	(5,743)
Amortization of subordinated debt issuance costs	437	438	230
Discount on repurchase of subordinated debt	(106)	0	0
Net change in other assets and other liabilities	(7,918)	(11,117)	(47,069)
Net cash provided by operating activities	183,223	159,185	145,273
Investing activities [Abstract]
Net cash provided by (used in) investing activities	(926,216)	(547,608)	(712,586)
Financing activities [Abstract]
Proceeds from issuance of subordinated debt	0	0	100,000
Payment of subordinated debt issuance costs	0	0	(2,178)
Repurchase of subordinated debt	(2,000)	0	0
Proceeds from the issuance of shares to employee and other stock plans	0	112	184
Cash paid by employer for tax-withholding on stock issuance	(1,751)	(2,931)	(1,537)
Purchases of treasury shares	(14,713)	(21,714)	(7,980)
Cash dividends	(49,765)	(47,738)	(47,207)
Net cash (used in) provided by financing activities	(328,728)	984,813	1,023,151
Net (decrease) increase in cash and cash equivalents	(1,071,721)	596,390	455,838
NBT Bancorp Inc [Member]
Operating activities [Abstract]
Net income	151,995	154,885	104,388
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	582	1,113	1,739
Excess tax benefit on stock-based compensation	(288)	(385)	(181)
Stock-based compensation expense	4,530	4,414	4,581
Net security losses (gains)	618	(543)	483
Equity in undistributed income of subsidiaries	(41,978)	(41,931)	(32,068)
Bank owned life insurance income	(238)	(326)	(391)
Amortization of subordinated debt issuance costs	437	438	230
Discount on repurchase of subordinated debt	(106)	0	0
Net change in other assets and other liabilities	(8,376)	(7,127)	(3,535)
Net cash provided by operating activities	107,176	110,538	75,246
Investing activities [Abstract]
Investment in the Bank	0	0	(90,000)
Proceeds from calls of equity securities	0	1,000	2,000
Net cash provided by (used in) investing activities	0	1,000	(88,000)
Financing activities [Abstract]
Proceeds from issuance of subordinated debt	0	0	100,000
Payment of subordinated debt issuance costs	0	0	(2,178)
Repurchase of subordinated debt	(2,000)	0	0
Proceeds from the issuance of shares to employee and other stock plans	0	112	184
Cash paid by employer for tax-withholding on stock issuance	(1,751)	(2,931)	(1,537)
Purchases of treasury shares	(14,713)	(21,714)	(7,980)
Cash dividends	(49,765)	(47,738)	(47,207)
Net cash (used in) provided by financing activities	(68,229)	(72,271)	41,282
Net (decrease) increase in cash and cash equivalents	38,947	39,267	28,528
Cash and cash equivalents at beginning of year	77,182	37,915	9,387
Cash and cash equivalents at end of year	$ 116,129	$ 77,182	$ 37,915