INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Held-to-Maturity Amortized Cost
Due in one year or less	$ 5,105
Due after one year through five years	563,324
Due after five years through ten years	133,886
Due after ten years	0
Total	702,315	$ 726,259
Held-to-Maturity Market Value
Due in one year or less	5,207
Due after one year through five years	574,597
Due after five years through ten years	138,473
Due after ten years	0
Held-to-Maturity Market Value	718,277	731,951
Available-for-Sale Amortized Cost
Due in one year or less	58,659
Due after one year through five years	775,602
Due after five years through ten years	303,552
Due after ten years	27,280
Total	1,165,093	1,203,065
Available-for-Sale Market Value
Due in one year or less	58,493
Due after one year through five years	774,467
Due after five years through ten years	304,623
Due after ten years	26,736
Available-for-Sale Market Value	$ 1,164,319	$ 1,190,642