SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Intangible Assets (Details)	1 Months Ended		12 Months Ended
	Nov. 30, 2013 Branch	Mar. 31, 2008 Branch	Dec. 31, 2015 USD ($)
M&T Bank branch offices
Finite-Lived Intangible Assets [Line Items]
Number of branches acquired | Branch		3
Finite-lived intangible assets			$ 0
Bank of America
Finite-Lived Intangible Assets [Line Items]
Number of branches acquired | Branch	6
Finite-lived intangible assets			$ 1,100,000
Bank of America | Core Deposits
Finite-Lived Intangible Assets [Line Items]
Useful life			7 years
Partners Trust Bank | Trust Business
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets			$ 2,000,000
Useful life			15 years
MT Branch Offices and Canton Bancorp, Inc. | Core Deposits
Finite-Lived Intangible Assets [Line Items]
Useful life			7 years 3 months
MT Branch Offices and Canton Bancorp, Inc. | Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Useful life			7 years 3 months
Canton Bancorp, Inc.
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets			$ 2,000
Fort Orange Financial Corp
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets			$ 800,000
Useful life			10 years