INVESTMENTS - Major Type - Maturity Period (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investments
Amortized Cost	$ 759,958	$ 824,074
Fair Value	759,725	824,186
Commercial paper | Less than 1 year
Investments
Amortized Cost	3,978	19,482
Fair Value	3,978	19,480
Municipal securities | Less than 1 year
Investments
Amortized Cost	709,207	744,542
Fair Value	709,078	744,647
Municipal securities | Due 1 - 10 years
Investments
Amortized Cost	11,071	42,940
Fair Value	11,063	42,950
U.S. government agency securities | Less than 1 year
Investments
Amortized Cost	23,369	9,199
Fair Value	23,311	9,198
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
Auction rate securities | Due 11 - 20 years
Investments
Amortized Cost		3,910
Fair Value		3,910
Variable rate demand notes | Due 21 - 30 years
Investments
Amortized Cost		4,001
Fair Value		$ 4,001