INVESTMENTS - Major Type - Maturity Period (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investments
Amortized Cost	$ 515,662	$ 759,958
Fair Value	515,523	759,725
Commercial paper | Less than 1 year
Investments
Amortized Cost	7,969	3,978
Fair Value	7,969	3,978
Municipal securities | Less than 1 year
Investments
Amortized Cost	464,865	709,207
Fair Value	464,739	709,078
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost	5,334	11,071
Fair Value	5,328	11,063
Municipal securities | Due 11 - 20 years
Investments
Amortized Cost	2,085
Fair Value	2,083
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	17,316	23,369
Fair Value	17,310	23,311
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost		4,277
Fair Value		4,252
U.S. Treasuries | Less than 1 year
Investments
Amortized Cost	18,093	8,056
Fair Value	$ 18,094	$ 8,043