INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Held-to-Maturity Amortized Cost
Due in one year or less	$ 4,473
Due after one year through five years	534,431
Due after five years through ten years	217,131
Due after ten years	0
Total	756,035	$ 867,996
Held-to-Maturity Market Value
Due in one year or less	4,587
Due after one year through five years	543,735
Due after five years through ten years	222,190
Due after ten years	0
Held-to-Maturity Market Value	770,512	874,749
Available-for-Sale Amortized Cost
Due in one year or less	22,066
Due after one year through five years	667,783
Due after five years through ten years	334,447
Due after ten years	47,262
Total	1,071,558	849,504
Available-for-Sale Market Value
Due in one year or less	22,118
Due after one year through five years	666,906
Due after five years through ten years	332,881
Due after ten years	47,762
Available-for-Sale Market Value	$ 1,069,667	$ 840,468