Parent Company Financial Information, Condensed Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities [Abstract]
Net income	$ 82,151	$ 78,409	$ 76,425
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	9,056	9,023	8,646
Excess tax benefit on stock-based compensation	1,769	1,055	(43)
Stock-based compensation expense	3,644	4,378	4,086
Net (gain) on sales of AFS securities	(1,869)	644	(3,087)
Re-evaluation of deferred tax amounts from Tax Act	4,407	0	0
Bank owned life insurance income	(5,175)	(5,195)	(4,334)
Net change in other liabilities	3,834	(10,697)	5,236
Net change in other assets	28	364	15,386
Net cash provided by operating activities	136,904	110,565	124,500
Investing activities [Abstract]
Proceeds on sales and maturities of AFS securities	290,613	324,781	299,302
Purchases of AFS securities	(233,804)	(597,428)	(481,262)
Proceeds from settlement of bank owned life insurance	799	1,477	1,541
Net purchases of premises and equipment	(6,691)	(3,308)	(8,193)
Net cash provided by (used in) investing activities	(305,193)	(629,960)	(504,004)
Financing activities [Abstract]
Proceeds from the issuance of shares to employee benefit plans and other stock plans	3,309	6,032	9,356
Cash paid by employer for tax-withholding on stock issuance	(3,582)	(3,387)	(1,664)
Purchases of treasury shares	0	(17,193)	(26,797)
Cash dividends and payments for fractional shares	(40,104)	(38,880)	(38,149)
Net cash provided by (used in) financing activities	178,772	528,279	373,165
Net increase (decrease) in cash and cash equivalents	10,483	8,884	(6,339)
Cash and cash equivalents at beginning of year	149,181	140,297	146,636
Cash and cash equivalents at end of year	159,664	149,181	140,297
NBT Bancorp Inc [Member]
Operating activities [Abstract]
Net income	82,151	78,409	76,425
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization of premises and equipment	2,974	2,805	2,522
Excess tax benefit on stock-based compensation	1,769	1,055	(43)
Stock-based compensation expense	3,644	4,378	4,086
Net (gain) on sales of AFS securities	(2,238)	(652)	(3,034)
Re-evaluation of deferred tax amounts from Tax Act	3,339	0	0
Equity in undistributed income of subsidiaries	(82,567)	(79,193)	(74,916)
Cash dividend from subsidiaries	38,300	10,200	78,200
Bank owned life insurance income	(328)	(356)	(292)
Net change in other liabilities	(5,624)	(8,596)	6,770
Net change in other assets	(368)	22,728	(6,652)
Net cash provided by operating activities	41,052	30,778	83,066
Investing activities [Abstract]
Proceeds on sales and maturities of AFS securities	4,710	1,783	5,297
Purchases of AFS securities	(9)	(580)	(3,083)
Proceeds from settlement of bank owned life insurance	308	0	0
Net purchases of premises and equipment	(2,264)	(3,083)	(2,930)
Net cash provided by (used in) investing activities	2,745	(1,880)	(716)
Financing activities [Abstract]
Proceeds from the issuance of shares to employee benefit plans and other stock plans	3,309	6,032	8,856
Cash paid by employer for tax-withholding on stock issuance	(3,582)	(3,387)	(1,164)
Purchases of treasury shares	0	(17,193)	(26,797)
Cash dividends and payments for fractional shares	(40,104)	(38,880)	(38,149)
Net cash provided by (used in) financing activities	(40,377)	(53,428)	(57,254)
Net increase (decrease) in cash and cash equivalents	3,420	(24,530)	25,096
Cash and cash equivalents at beginning of year	4,152	28,682	3,586
Cash and cash equivalents at end of year	$ 7,572	$ 4,152	$ 28,682