ACQUISITIONS (Details) (USD $)	3 Months Ended		9 Months Ended				9 Months Ended		0 Months Ended				9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Apr. 08, 2011	Sep. 30, 2012 Core Deposits [Member]	Sep. 30, 2012 Other Intangibles [Member]	Apr. 08, 2011 Wilber Corporation [Member]	Apr. 08, 2011 Wilber Corporation [Member] Acquired Impaired Loans [Member]	Apr. 08, 2011 Wilber Corporation [Member] Acquired Non-Impaired Loans [Member]	Nov. 30, 2011 Combined CAI Benefits and Wilber Corporation Acquisitions [Member]	Sep. 30, 2012 HSBC and First Niagara [Member]	Sep. 30, 2012 HSBC and First Niagara [Member] Acquired Impaired Loans [Member]	Sep. 30, 2012 HSBC and First Niagara [Member] Acquired Non-Impaired Loans [Member]	Jul. 20, 2012 HSBC Branch Agreement [Member]	Sep. 07, 2012 First Niagara Branch Agreement [Member]
Business Acquisition [Line Items]
Number of new branch locations included in acquisition or purchase agreement									22							16	3
Loans acquired																$ 106,000,000	$ 54,000,000
Deposits acquired																697,000,000	101,000,000
Blended deposit premium (in hundredths)																3.40%	3.10%
Blended deposit premium paid																24,000,000	3,000,000
Public stock offering:
Amount raised through issuance			62,873,000	2,273,000									57,500,000
Number of shares issued (in shares)			2,130,000
Net proceeds from offering			54,917,000
Business Acquisition, Description of Acquired Entity			certain assets and liabilities of CAI Benefits, Inc. ("CAI"), a provider of actuarial, consulting and retirement plan administration services, with offices in New York City and Northern New Jersey.
Assets acquired												1,400,000
Liabilities acquired												200,000
Total consideration paid (received)	(595,462,000)		(595,462,000)		104,465,000				103,000,000
Cash portion of purchase price	(595,462,000)		(595,462,000)		21,885,000				20,400,000
Issuance of shares of common stock (in shares)									3,350,000
Loans	160,116,000		160,116,000		462,334,000	462,300,000
Investment securities	0		0		297,573,000	297,600,000
Deposits						771,600,000
Borrowings	0		0		19,668,000	19,700,000
Merger and acquisition integration related expenses	4,800,000	400,000	5,200,000	4,700,000
Consideration paid(received):
Community Bank System, Inc. common stock	0		0		82,580,000
Cash	(595,462,000)		(595,462,000)		21,885,000				20,400,000
Total consideration paid (received)	(595,462,000)		(595,462,000)		104,465,000				103,000,000
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents	5,510,000		5,510,000		26,901,000
Investment securities	0		0		297,573,000	297,600,000
Loans	160,116,000		160,116,000		462,334,000	462,300,000
Premises and equipment	4,941,000		4,941,000		6,353,000
Accrued interest receivable	588,000		588,000		2,615,000
Other assets and liabilities, net	171,000		171,000		46,942,000
Core deposit intangibles	6,521,000		6,521,000		4,016,000
Other intangibles	0		0		1,595,000
Deposits	(797,962,000)		(797,962,000)		(771,554,000)
Borrowings	0		0		(19,668,000)	(19,700,000)
Total identifiable assets and liabilities, net	(620,115,000)		(620,115,000)		57,107,000
Goodwill	24,653,000		24,653,000		47,358,000
Summary of loans acquired [Abstract]
Contractually required principal and interest at acquisition									722,246,000	41,730,000	680,516,000		201,745,000	0	201,745,000
Contractual cash flows not expected to be collected									(51,176,000)	(20,061,000)	(31,115,000)		(3,555,000)	0	(3,555,000)
Expected cash flows at acquisition									671,070,000	21,669,000	649,401,000		198,190,000	0	198,190,000
Interest component of expected cash flows									(208,736,000)	(2,509,000)	(206,227,000)		(37,473,000)	0	(38,074,000)
Fair value of acquired loans									462,334,000	19,160,000	443,174,000		160,717,000	0	160,116,000
Unaudited proforma information [Abstract]
Total revenue, net of interest expense
Net income
Basic
Diluted
Finite-Lived Intangible Assets [Line Items]
Estimated useful life of core deposit intangible and other intangible, minimum (in years)			8				8	8
Estimated useful life of core deposit intangible and other intangible, maximum (in years)			20				10	10