ACQUISITIONS (Details) (USD $)	12 Months Ended									0 Months Ended			12 Months Ended	1 Months Ended	12 Months Ended						12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Maximum [Member] Core Deposits [Member]	Dec. 31, 2013 Maximum [Member] Core Deposits From Acquisitions [Member]	Dec. 31, 2013 Maximum [Member] Other Intangibles [Member]	Dec. 31, 2013 Minimum [Member] Core Deposits [Member]	Dec. 31, 2013 Minimum [Member] Core Deposits From Acquisitions [Member]	Dec. 31, 2013 Minimum [Member] Other Intangibles [Member]	Apr. 08, 2011 Wilber Corporation [Member] Branch	Apr. 08, 2011 Wilber Corporation [Member] Acquired Impaired Loans [Member]	Apr. 08, 2011 Wilber Corporation [Member] Acquired Non-Impaired Loans [Member]	Dec. 31, 2011 Combined CAI Benefits and Wilber Corporation Acquisitions [Member]	Nov. 30, 2011 CAI Benefits [Member]	Dec. 31, 2012 HSBC and First Niagara [Member]	Jul. 20, 2012 HSBC and First Niagara [Member]	Jul. 20, 2012 HSBC and First Niagara [Member] Acquired Impaired Loans [Member]	Jul. 20, 2012 HSBC and First Niagara [Member] Acquired Non-Impaired Loans [Member]	Jul. 20, 2012 HSBC Branch Agreement [Member] Branch	Sep. 07, 2012 First Niagara Branch Agreement [Member] Branch	Dec. 31, 2013 Bank of America Agreement [Member]	Dec. 13, 2013 Bank of America Agreement [Member] Branch
Business Acquisition [Line Items]
Number of new branch locations included in acquisition or purchase agreement										22									16	3		8
Loans acquired										$ 462,300,000									$ 106,000,000	$ 54,000,000		$ 1,000,000
Deposits acquired																			697,000,000	101,000,000		303,000,000
Blended deposit premium (in hundredths)																			3.40%	3.10%		2.40%
Total deposit premium paid																			24,000,000	3,000,000		7,300,000
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
Issuance of shares of common stock (in shares)										3,350,000
Cash portion of purchase price										20,400,000			22,155,000		(595,462,000)						(291,980,000)
Loans										462,300,000			462,334,000		160,116,000						1,106,000
Investment securities										297,600,000			297,573,000		0						0
Deposits										771,600,000			771,554,000		797,962,000						303,456,000
Borrowings										19,700,000			19,668,000		0						0
Consideration received [Abstract]
Community Bank System, Inc. common stock													82,580,000		0						0
Cash portion of purchase price										20,400,000			22,155,000		(595,462,000)						(291,980,000)
Total net consideration paid (received)										103,000,000			104,735,000		(595,462,000)						(291,980,000)
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents													26,901,000		5,510,000						0
Investment securities										297,600,000			297,573,000		0						0
Loans										462,300,000			462,334,000		160,116,000						1,106,000
Premises and equipment													6,360,000		4,941,000						2,549,000
Accrued interest receivable													2,615,000		588,000						5,000
Other assets/(liabilities), net													46,942,000		171,000						(18,000)
Core deposit intangibles													4,016,000		6,521,000						2,537,000
Other intangibles													1,858,000		0						9,000
Deposits										(771,600,000)			(771,554,000)		(797,962,000)						(303,456,000)
Borrowings										(19,700,000)			(19,668,000)		0						0
Total identifiable assets/(liabilities), net													57,377,000		(620,115,000)						(297,268,000)
Goodwill	374,991,000	369,703,000	345,050,000										47,358,000		24,653,000						5,288,000
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
Merger and acquisition integration related expenses	$ 2,200,000	$ 5,700,000	$ 4,800,000
Finite-Lived Intangible Assets [Line Items]
Estimated useful life of core deposit intangible and other intangible				20 years	10 years	10 years	7 years	8 years	8 years