BUSINESS COMBINATIONS - Pro Forma Information (Details) - Coastal Bankshares, Inc. - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Net interest income and noninterest income	$ 514,885	$ 477,500
Net income	134,486	97,686
Net income available to common shareholders	$ 134,486	$ 97,686
Income per common share available to common shareholders – basic (in dollars per share)	$ 2.83	$ 2.08
Income per common share available to common shareholders – diluted (in dollars per share)	$ 2.83	$ 2.07
Average number of shares outstanding, basic (in shares)	47,460,000	46,959,000
Average number of shares outstanding, diluted (in shares)	47,566,000	47,275,000