ACQUISITION - Narrative (Details) $ / shares in Units, shares in Thousands		3 Months Ended
	Jan. 13, 2017 USD ($) banking_office subsidiary $ / shares shares	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Business Acquisition [Line Items]
Pro forma estimated tax rate		37.57%
Lake Sunapee
Business Acquisition [Line Items]
Number of subsidiaries | subsidiary	1
Number of full service branches acquired | banking_office	33
Common shares outstanding | shares	8,380
Shares issued/exchanged in acquisition | shares	4,164
Fixed exchange ratio	0.4970
Acquired price per share (in dollars per share) | $ / shares	$ 43.69
Total consideration paid	$ 181,900,000	$ 181,946,000
Cash paid for fractional shares	$ 27,000
Acquisition integration related costs		$ 3,100,000	$ 0