ACQUISITIONS (Details) $ in Thousands		3 Months Ended	9 Months Ended		12 Months Ended
	Feb. 24, 2015 USD ($) Branch	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill		$ 375,174	$ 375,174	$ 375,174	$ 375,174
Merger and acquisition integration-related expenses		$ 600	$ 1,300	$ 100
Other Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Estimated useful life of core deposit intangible and other intangible		8 years
EBS-RMSCO, Inc. [Member]
Consideration paid [Abstract]
Cash/Total net consideration paid					924
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Other assets					163
Other intangibles					578
Total identifiable assets					741
Goodwill					$ 183
Oneida Financial Corp [Member]
Consideration paid [Abstract]
Cash/Total net consideration paid	$ 142,000
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Number of new branch locations included in acquisition or purchase agreement | Branch	12
Assets acquired	$ 813,000
Loans acquired	397,000
Deposits acquired	$ 708,000