SECURITIES - Amortized Cost and Estimated Fair Value of Debt Securities, Available for Sale, by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 11,510
Due from one year to five years	58,549
Due from five to ten years	73,566
Due after ten years	39,002
Mortgage-backed securities	630,666
Amortized Cost	813,293	$ 824,635
Estimated Fair Value
Due in one year or less	11,562
Due from one year to five years	58,881
Due from five to ten years	74,861
Due after ten years	39,633
Mortgage-backed securities	625,936
Estimated Fair Value	$ 810,873	$ 822,735