ACQUISITION - Pro Forma Information (Details) - Lake Sunapee - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Net interest income	$ 93,200	$ 90,539
Non-interest income	26,072	32,484
Net income	$ 33,100	$ 27,084
Pro forma earnings per share, basic (in dollars per share)	$ 2.18	$ 1.77
Pro forma earnings per share, diluted (in dollars per share)	$ 2.16	$ 1.76