ACQUISITIONS (Details) (USD $) In Thousands, except Share data in Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Apr. 08, 2011
Loans acquired in Wilber acquisition [Abstract]
Contractually required principal and interest at acquisition			$ 722,246
Contractual cash flows not expected to be collected			(51,176)
Expected cash flows at acquisition			671,070
Interest component of expected cash flows			(208,736)
Fair value of acquired loans			462,334
Accelerated method amortization period (in years)	8
Merger and acquisition integration related expenses	4,831
Acquired Impaired Loans [Member]
Loans acquired in Wilber acquisition [Abstract]
Contractually required principal and interest at acquisition			41,730
Contractual cash flows not expected to be collected			(20,061)
Expected cash flows at acquisition			21,669
Interest component of expected cash flows			(2,509)
Fair value of acquired loans			19,160
Acquired Non-Impaired Loans [Member]
Loans acquired in Wilber acquisition [Abstract]
Contractually required principal and interest at acquisition			680,516
Contractual cash flows not expected to be collected			(31,115)
Expected cash flows at acquisition			649,401
Interest component of expected cash flows			(206,227)
Fair value of acquired loans			443,174
Wilber Acquisition [Member]
Business Acquisition [Line Items]
Acquisition purchase price			103,000
Business Acquisition, Description of Acquired Entity	Based in Oneonta, New York, Wilber operated 22 branches in the Central, Greater Capital District, and Catskill regions of Upstate New York.
Number of new branch locations included in acquisition agreement			22
Cash portion of purchase price			20,400
Issuance of shares of common stock (in shares)	3.35
Consideration paid:
Cash			20,400
Total consideration paid			103,000
Business Acquisition, Description of Acquired Entity	Based in Oneonta, New York, Wilber operated 22 branches in the Central, Greater Capital District, and Catskill regions of Upstate New York.
Number of new branch locations included in acquisition agreement			22
Cash portion of purchase price			20,400
Issuance of shares of common stock (in shares)	3.35
Supplemental pro forma information [Abstract]
Total revenue, net of interest expense	308,559	314,425
Net income	70,424	68,176
Earnings per share:
Basic earnings per share (in dollars per share)	$ 1.92	$ 1.88
Diluted earnings per share (in dollars per share)	$ 1.89	$ 1.85
Combined CAI Benefits and Wilber Corporation Acquisitions [Member]
Business Acquisition [Line Items]
Acquisition purchase price	104,735
Cash portion of purchase price	22,155
Consideration paid:
Community Bank System, Inc. common stock	82,580
Cash	22,155
Total consideration paid	104,735
Cash portion of purchase price	22,155
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents	26,901
Investment securities	297,573
Loans	462,334
Premises and equipment	6,360
Accrued interest receivable	2,615
Other assets and liabilities, net	46,943
Core deposit intangibles	4,015
Other intangibles	1,858
Deposits	(771,554)
Borrowings	(19,668)
Total identifiable assets	57,377
Goodwill	$ 47,358