INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Held-to-Maturity Amortized Cost
Due in one year or less	$ 5,426
Due after one year through five years	360,042
Due after five years through ten years	244,118
Due after ten years	230,080
Total	839,666	867,996
Held-to-Maturity Market Value
Due in one year or less	5,545
Due after one year through five years	364,027
Due after five years through ten years	249,477
Due after ten years	236,034
Held-to-Maturity Market Value	855,083	874,749
Available-for-Sale Amortized Cost
Due in one year or less	25,034
Due after one year through five years	365,068
Due after five years through ten years	167,204
Due after ten years	335,817
Total	893,123	849,504
Available-for-Sale Market Value
Due in one year or less	25,125
Due after one year through five years	367,463
Due after five years through ten years	168,454
Due after ten years	331,127
Available-for-Sale Market Value	$ 892,169	$ 840,468