INVESTMENTS - Major Type - Maturity Period (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investments
Amortized Cost	$ 223,144	$ 759,958
Fair Value	222,948	759,725
Commercial paper | Less than 1 year
Investments
Amortized Cost	40,382	3,978
Fair Value	40,382	3,978
Municipal securities | Less than 1 year
Investments
Amortized Cost	140,379	709,207
Fair Value	140,198	709,078
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost	2,403	11,071
Fair Value	2,394	11,063
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	26,057	23,369
Fair Value	26,051	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
Variable rate demand notes | Due 1 - 10 years
Investments
Amortized Cost	3,917
Fair Value	3,917
Variable rate demand notes | Due 11 - 20 years
Investments
Amortized Cost	6,003
Fair Value	6,003
Variable rate demand notes | Due 21 - 30 years
Investments
Amortized Cost	4,003
Fair Value	$ 4,003