ACQUISITIONS (Details) (USD $)	12 Months Ended		12 Months Ended		0 Months Ended				1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Core Deposits [Member]	Dec. 31, 2012 Other Intangibles [Member]	Apr. 08, 2011 Wilber Corporation [Member]	Dec. 31, 2012 Wilber Corporation [Member]	Dec. 31, 2012 Wilber Corporation [Member] Acquired Impaired Loans [Member]	Dec. 31, 2012 Wilber Corporation [Member] Acquired Non-Impaired Loans [Member]	Nov. 30, 2011 Combined CAI Benefits and Wilber Corporation Acquisitions [Member]	Dec. 31, 2012 HSBC and First Niagara [Member]	Dec. 31, 2012 HSBC and First Niagara [Member] Acquired Impaired Loans [Member]	Dec. 31, 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					$ 462,000,000								$ 106,000,000	$ 54,000,000
Deposits acquired													697,000,000	101,000,000
Blended deposit premium (in hundredths)													3.40%	3.10%
Total deposit premium paid													24,000,000	3,000,000
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
Cash portion of purchase price	(595,462,000)	22,155,000			20,400,000
Issuance of shares of common stock (in shares)					3,350,000
Loans	160,116,000	462,334,000			462,300,000
Investment securities	0	297,573,000			297,600,000
Deposits	797,962,000	771,554,000			771,600,000
Borrowings	0	19,668,000			19,700,000
Consideration paid (received) [Abstract]
Community Bank System, Inc. common stock	0	82,580,000
Cash	(595,462,000)	22,155,000			20,400,000
Total consideration paid (received)	(595,462,000)	104,735,000			103,000,000
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	5,510,000	26,901,000
Investment securities	0	297,573,000			297,600,000
Loans	160,116,000	462,334,000			462,300,000
Premises and equipment	4,941,000	6,360,000
Accrued interest receivable	588,000	2,615,000
Other assets and liabilities, net	171,000	46,942,000
Core deposit intangibles	6,521,000	4,016,000
Other intangibles	0	1,858,000
Deposits	(797,962,000)	(771,554,000)			(771,600,000)
Borrowings	0	(19,668,000)			(19,700,000)
Total identifiable assets and liabilities, net	(620,115,000)	57,377,000
Goodwill	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)		(38,074,000)	0	(38,074,000)
Fair value of acquired loans						462,334,000	19,160,000	443,174,000		160,116,000	0	160,116,000
Direct cost for acquisition	$ 5,700,000	$ 4,800,000
Finite-Lived Intangible Assets [Line Items]
Estimated useful life of core deposit intangible and other intangible, minimum (in years)	7		8	8
Estimated useful life of core deposit intangible and other intangible, maximum (in years)	20		10	10