BUSINESS COMBINATIONS - Schedule of Pro Forma Information (Details) - Hamilton, Atlantic, And USPF - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Net interest income and noninterest income	$ 129,209	$ 121,864	$ 384,861	$ 355,813
Net income	41,444	27,668	90,950	87,311
Net income available to common shareholders	$ 41,444	$ 27,668	$ 90,950	$ 87,311
Income per common share available to common shareholders – basic (in dollars per share)	$ 0.87	$ 0.58	$ 1.92	$ 1.86
Income per common share available to common shareholders – diluted (in dollars per share)	$ 0.87	$ 0.58	$ 1.91	$ 1.85
Average number of shares outstanding, basic (in shares)	47,515	47,350	47,447	46,822
Average number of shares outstanding, diluted (in shares)	47,685	47,677	47,619	47,150