INVESTMENTS (Details 2) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investments
Amortized Cost	$ 1,287,222	$ 824,074
Fair Value	1,287,313	824,186
U.S. Treasuries
Investments
Amortized Cost	74,981
Fair Value	74,987
Certificates of deposit
Investments
Amortized Cost	32,001
Fair Value	32,001
Commercial paper | Less than 1 year
Investments
Amortized Cost	182,130	19,482
Fair Value	182,130	19,480
Municipal securities | Less than 1 year
Investments
Amortized Cost	587,740	744,542
Fair Value	587,780	744,647
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost	52,364	42,940
Fair Value	52,392	42,950
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	350,859	9,199
Fair Value	350,876	9,198
Variable rate demand notes | Due 21 - 30 years
Investments
Amortized Cost	3,901	4,001
Fair Value	3,901	4,001
Auction rate securities | Due 11 - 20 years
Investments
Amortized Cost	3,246	3,910
Fair Value	$ 3,246	$ 3,910