COMMITMENTS AND CONTINGENCIES - Schedule of Guarantor Obligations (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments to extend credit	$ 1,581,342	$ 1,109,806
Unused home equity lines of credit	112,706	69,788
Financial standby letters of credit	21,862	11,389
Mortgage interest rate lock commitments	113,642	86,149
Mortgage forward contracts with positive fair value	$ 246,601	$ 31,500