SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
INCOME TAXES [Abstract]
Tax position recognition criteria percentage above which is "more likely than not" (in hundredths)	50.00%
WEALTH MANAGEMENT GROUP FEE INCOME [Abstract]
Assets under management, carrying amount	$ 1,735,000,000	$ 1,596,000,000
Defined Benefit Plan Disclosure [Line Items]
Normal retirement benefit	1.20%
Number of consecutive years considered in final average compensation	5 years
Number of prior years considered in final average compensation	10 years
Maximum service period	25 years
Normal retirement additional benefit (in hundredths)	1.00%
Maximum service years	35 years
Percentage of average monthly compensation (in hundredths)	0.65%
Maximum period of credited service	35 years
Covered compensation period	35 years
Minimum annual service period	1000 hours
Minimum eligible age (in years)	55
Contribution percentage (in hundredths)	40.00%
Retiree premium percentage (in hundredths)	100.00%
Cost of coverage contribution (in hundredths)	50.00%
Contribution percentage after amendment (in hundredths)	50.00%
Contribution cost percentage (in hundredths)	50.00%
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock award vesting percentage (in hundredths)	20.00%
Restricted stock award vesting percentage, total (in hundredths)	100.00%
Age limit for forfeiture of stock award (in years)	65
Non employee directors service period	12 months
Trading days period	30 days
Cash compensation	300,000
Maximum annual installments period	10 years
Segment Reporting [Abstract]
Number of operating segments	2
M&T Bank branch offices [Member]
Intangible Assets [Abstract]
Intangible assets with definite useful lives - from business purchases	198,819
Finite-lived intangible asset, useful life	7 years 3 months
Partners Trust Bank [Member]
Intangible Assets [Abstract]
Intangible assets with definite useful lives - from business purchases	3,073,504
Finite-lived intangible asset, useful life	15 years
Canton Bancorp, Inc [Member]
Intangible Assets [Abstract]
Intangible assets with definite useful lives - from business purchases	31,654
Finite-lived intangible asset, useful life	7 years
Fort Orange Financial Corp [Member]
Intangible Assets [Abstract]
Intangible assets with definite useful lives - from business purchases	$ 1,839,843
Finite-lived intangible asset, useful life	10 years
Directors' Compensation Plan [Member] | Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized per calendar year (in shares)	20,000
Incentive Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized per calendar year (in shares)	10,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized per calendar year (in shares)	15,000
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Retiree age group range (in years)	55
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Retiree age group range (in years)	65
Building [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	15 years
Building [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	50 years
Equipment and furniture [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Equipment and furniture [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years