INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments
Amortized Cost	$ 211,207	$ 676,129
Fair Value	211,093	675,299
Commercial paper | Less than 1 year
Investments
Amortized Cost	15,177	81,026
Fair Value	15,177	81,026
Municipal securities | Less than 1 year
Investments
Amortized Cost	149,048	469,604
Fair Value	148,997	468,865
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	46,982	61,307
Fair Value	$ 46,919	61,219
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost		2,369
Fair Value		2,366
Certificates of deposit | Less than 1 year
Investments
Amortized Cost		11,869
Fair Value		11,869
Variable rate demand notes | Due 1 - 10 years
Investments
Amortized Cost		6,366
Fair Value		6,366
Variable rate demand notes | Due 11 - 20 years
Investments
Amortized Cost		28,377
Fair Value		28,377
Variable rate demand notes | Due 21 - 30 years
Investments
Amortized Cost		15,211
Fair Value		$ 15,211