INVESTMENTS - Maturity Period (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investments
Amortized Cost	$ 372,582	$ 223,144
Fair Value	372,490	222,948
Commercial paper | Less than 1 year
Investments
Amortized Cost	73,037	40,382
Fair Value	73,040	40,382
Certificates of deposit | Less than 1 year
Investments
Amortized Cost	37,210
Fair Value	37,210
Municipal securities | Less than 1 year
Investments
Amortized Cost	152,287	140,379
Fair Value	152,253	140,198
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost	32,704	2,403
Fair Value	32,687	2,394
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	28,465	26,057
Fair Value	28,443	26,051
U.S. government agency securities | Due 1 - 10 years
Investments
Amortized Cost	15,974
Fair Value	15,952
Variable rate demand notes | Due 1 - 10 years
Investments
Amortized Cost	3,918	3,917
Fair Value	3,918	3,917
Variable rate demand notes | Due 11 - 20 years
Investments
Amortized Cost	10,006	6,003
Fair Value	10,006	6,003
Variable rate demand notes | Due 21 - 30 years
Investments
Amortized Cost	14,979	4,003
Fair Value	14,979	$ 4,003
Variable rate demand notes | Due 31 - 40 years
Investments
Amortized Cost	4,002
Fair Value	$ 4,002