INVESTMENT SECURITIES - Amortized Cost and Fair Value of Available for Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 12,384
Due from one year to five years	85,744
Due from five to ten years	95,997
Due after ten years	31,646
Mortgage-backed securities	944,710
Amortized Cost	1,170,481	$ 813,293
Estimated Fair Value
Due in one year or less	12,407
Due from one year to five years	85,238
Due from five to ten years	96,826
Due after ten years	31,756
Mortgage-backed securities	927,476
Estimated Fair Value	$ 1,153,703	$ 810,873