Parent Company Financial Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Assets
Cash and cash equivalents	$ 129,381	$ 168,792	$ 187,161
Securities available for sale, at estimated fair value	1,244,619	1,129,368
Trading securities	3,062	2,808
Other assets	92,254	99,471
Total assets	5,598,406	5,338,856
Liabilities and Stockholders' Equity
Total liabilities	5,060,296	4,805,284
Stockholders' equity	538,110	533,572	505,123	431,845
Total liabilities and stockholders' equity	5,598,406	5,338,856
Securities gains	150	3,274	144
Interest, dividend and other income	239,997	255,738	273,393
Income before income tax benefit and equity in undistributed income of subsidiaries	79,174	78,316	72,642
Income tax benefit	(21,273)	(20,912)	(20,631)
Net income	57,901	57,404	52,011
Operating activities
Net income	57,901	57,404	52,011
Adjustments to reconcile net income to net cash provided by operating activities
Stock-based compensation	3,244	3,782	3,133
Gain on sales of available-for-sale securities	(7)	(3,170)	(154)
Net change in other liabilities	11,805	2,645	6,399
Net change in other assets	(3,579)	6,627	(39,324)
Net cash provided by operating activities	83,765	89,437	54,684
Investing activities
Purchases of available-for-sale securities	(648,048)	(635,319)	(426,979)
Sales and maturities of available-for-sale securities	541,555	511,394	434,127
Purchases of premises and equipment	(9,954)	(6,510)	(6,378)
Net cash (used in) provided by investing activities	(175,498)	58,494	(28,448)
Financing activities
Payments on long-term debt	(2,146)	(184,824)	(77,511)
Proceeds from the issuance of common stock	0	0	33,401
Purchases of treasury shares	30,502	477
Cash dividends and payments for fractional shares	(27,063)	(27,577)	(27,119)
Excess tax benefit from exercise of stock options	341	140	(243)
Net cash provided by (used in) financing activities	52,322	(166,300)	50,529
Net (decrease) increase in cash and cash equivalents	(39,411)	(18,369)	76,765
Cash and cash equivalents at beginning of year	168,792	187,161	110,396
Cash and cash equivalents at end of year	129,381	168,792	187,161
NBT Bancorp Inc [Member]
Assets
Cash and cash equivalents	2,454	3,493	11,584
Securities available for sale, at estimated fair value	9,825	9,190
Trading securities	2,550	2,273
Investment in subsidiaries, on equity basis	622,885	609,260
Other assets	48,153	41,942
Total assets	685,867	666,158
Liabilities and Stockholders' Equity
Total liabilities	147,757	132,586
Stockholders' equity	538,110	533,572
Total liabilities and stockholders' equity	685,867	666,158
Dividends from subsidiaries	54,400	12,000	9,000
Management fee from subsidiaries	69,430	68,228	65,596
Securities gains	(31)	1,814	141
Interest, dividend and other income	628	1,136	869
Total revenue	124,427	83,178	75,606
Operating expense	75,254	72,436	73,687
Income before income tax benefit and equity in undistributed income of subsidiaries	49,173	10,742	1,919
Income tax benefit	1,340	282	1,994
Equity in undistributed income of subsidiaries	7,388	46,380	48,098
Net income	57,901	57,404	52,011
Operating activities
Net income	57,901	57,404	52,011
Adjustments to reconcile net income to net cash provided by operating activities
Stock-based compensation	3,244	3,782	3,133
Gain on sales of available-for-sale securities	31	(1,814)	(141)
Equity in undistributed income of subsidiaries	(7,388)	(46,380)	(48,098)
Net change in other liabilities	15,311	2,155	(3,662)
Net change in other assets	(11,607)	(2,605)	3,632
Net cash provided by operating activities	57,492	12,542	6,875
Investing activities
Purchases of available-for-sale securities	(968)	(1,745)	(2,173)
Sales and maturities of available-for-sale securities	71	7,807	494
Purchases of premises and equipment	(1,656)	(321)	(600)
Net cash (used in) provided by investing activities	(2,553)	5,741	(2,279)
Financing activities
Proceeds from the issuance of shares to employee benefit plans and other stock plans	1,386	1,675	2,728
Payments on long-term debt	(140)	(135)	(12,625)
Proceeds from the issuance of common stock	0	0	33,401
Purchases of treasury shares	(30,502)	(477)	0
Cash dividends and payments for fractional shares	(27,063)	(27,577)	(27,119)
Excess tax benefit from exercise of stock options	341	140	(243)
Net cash provided by (used in) financing activities	(55,978)	(26,374)	(3,858)
Net (decrease) increase in cash and cash equivalents	(1,039)	(8,091)	738
Cash and cash equivalents at beginning of year	3,493	11,584	10,846
Cash and cash equivalents at end of year	$ 2,454	$ 3,493	$ 11,584