INVESTMENTS - Major Type - Maturity Period (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investments
Amortized Cost	$ 44,319	$ 759,958
Fair Value	44,317	759,725
Commercial paper | Less than 1 year
Investments
Amortized Cost		3,978
Fair Value		3,978
Municipal securities | Less than 1 year
Investments
Amortized Cost	44,319	709,207
Fair Value	$ 44,317	709,078
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost		11,071
Fair Value		11,063
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost		23,369
Fair Value		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		8,056
Fair Value		$ 8,043