INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Held-to-Maturity Amortized Cost
Due in one year or less	$ 279
Due after one year through five years	574,250
Due after five years through ten years	98,462
Due after ten years	43,223
Total	716,214	770,755
Held-to-Maturity Market Value
Due in one year or less	283
Due after one year through five years	578,018
Due after five years through ten years	100,884
Due after ten years	44,041
Held-to-Maturity Market Value	723,226	778,474
Available-for-Sale Amortized Cost
Due in one year or less	10,129
Due after one year through five years	393,101
Due after five years through ten years	242,072
Due after ten years	297,697
Total	942,999	1,017,104
Available-for-Sale Market Value
Due in one year or less	10,604
Due after one year through five years	400,391
Due after five years through ten years	243,406
Due after ten years	297,638
Available-for-Sale Market Value	$ 952,039	$ 1,032,096