INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments
Amortized Cost	$ 589,337	$ 676,129
Fair Value	588,719	675,299
Commercial paper | Less than 1 year
Investments
Amortized Cost	42,288	81,026
Fair Value	42,288	81,026
Municipal securities | Less than 1 year
Investments
Amortized Cost	441,360	469,604
Fair Value	440,852	468,865
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost	527
Fair Value	527
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	74,773	61,307
Fair Value	74,666	61,219
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost	2,973	2,369
Fair Value	2,970	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	6,622	6,366
Fair Value	6,622	6,366
Variable rate demand notes | Due 11 - 20 years
Investments
Amortized Cost	18,542	28,377
Fair Value	18,542	28,377
Variable rate demand notes | Due 21 - 30 years
Investments
Amortized Cost	2,252	15,211
Fair Value	$ 2,252	$ 15,211