REGULATORY MATTERS (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Total core capital to risk-weighted assets [Abstract]
Minimum total core capital to risk-weighted assets for capital adequacy (in hundredths)	8.00%
Minimum Tier I capital to risk-weighted assets for capital adequacy (in hundredths)	4.00%
Minimum Tier I capital to average assets for capital adequacy (in hundredths)	4.00%
Minimum total core capital to risk-weighted assets to be well capitalized (in hundredths)	10.00%
Minimum Tier I capital to risk-weighted assets to be well capitalized (in hundredths)	6.00%
Minimum Tier I capital to average assets to be well capitalized (in hundredths)	5.00%
Community Bank System, Inc [Member]
Tier 1 capital to average assets [Abstract]
Tier 1 Capital	$ 508,250	$ 424,408
Tier 1 capital to average assets (in hundredths)	8.38%	8.23%
Minimum required amount	242,559	206,206
Tier 1 capital to risk-weighted assets [Abstract]
Tier 1 Capital	508,250	424,408
Tier 1 capital to risk-weighted assets (in hundredths)	14.31%	13.49%
Minimum required amount	142,074	125,840
Total core capital to risk-weighted assets [Abstract]
Core Capital	550,763	463,777
Total core capital to risk-weighted assets (in hundredths)	15.51%	14.74%
Minimum required amount	284,148	251,680
Community Bank, N.A. [Member]
Tier 1 capital to average assets [Abstract]
Tier 1 Capital	459,680	380,174
Tier 1 capital to average assets (in hundredths)	7.60%	7.40%
Minimum required amount	241,995	205,622
Tier 1 capital to risk-weighted assets [Abstract]
Tier 1 Capital	502,193	380,174
Tier 1 capital to risk-weighted assets (in hundredths)	12.99%	12.13%
Minimum required amount	141,529	125,404
Total core capital to risk-weighted assets [Abstract]
Core Capital	502,193	419,408
Total core capital to risk-weighted assets (in hundredths)	14.19%	13.38%
Minimum required amount	$ 283,058	$ 250,809