BUSINESS COMBINATIONS - Pro Forma Information (Details) - Coastal Bankshares, Inc. - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Net interest income and noninterest income	$ 329,248	$ 286,573
Net income	72,835	47,994
Net income available to common shareholders	$ 72,835	$ 47,994
Income per common share available to common shareholders – basic (in dollars per share)	$ 2.1	$ 1.40
Income per common share available to common shareholders – diluted (in dollars per share)	$ 2.07	$ 1.38
Average number of shares outstanding, basic (in shares)	34,841	34,311
Average number of shares outstanding, diluted (in shares)	35,196	34,676