INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments
Amortized Cost	$ 676,129	$ 223,144
Fair Value	675,299	222,948
Commercial paper | Less than 1 year
Investments
Amortized Cost	81,026	40,382
Fair Value	81,026	40,382
Municipal securities | Less than 1 year
Investments
Amortized Cost	469,604	140,379
Fair Value	468,865	140,198
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost		2,403
Fair Value		2,394
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	61,307	26,057
Fair Value	61,219	26,051
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	3,917
Fair Value	6,366	3,917
Variable rate demand notes | Due 11 - 20 years
Investments
Amortized Cost	28,377	6,003
Fair Value	28,377	6,003
Variable rate demand notes | Due 21 - 30 years
Investments
Amortized Cost	15,211	4,003
Fair Value	$ 15,211	$ 4,003