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INVESTMENTS (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale
Amortized Cost	$ 211,207	$ 676,129
Gross Unrealized Holding Gains	5	1
Gross Unrealized Holding Losses	119	831
Fair Value	211,093	675,299
Continuous Unrealized Loss Position less than 12 Months	119	831
Short-term | Commercial paper
Available-for-sale
Amortized Cost	15,177	81,026
Fair Value	15,177	81,026
Short-term | Certificates of deposit
Available-for-sale
Amortized Cost		11,869
Fair Value		11,869
Short-term | Municipal securities
Available-for-sale
Amortized Cost	149,048	469,604
Gross Unrealized Holding Gains	5	1
Gross Unrealized Holding Losses	56	740
Fair Value	148,997	468,865
Continuous Unrealized Loss Position less than 12 Months	56	740
Short-term | U.S. government agency securities
Available-for-sale
Amortized Cost	46,982	61,307
Gross Unrealized Holding Losses	63	88
Fair Value	46,919	61,219
Continuous Unrealized Loss Position less than 12 Months	$ 63	88
Short-term | Variable rate demand notes
Available-for-sale
Amortized Cost		49,954
Fair Value		49,954
Long-term | U.S. government agency securities
Available-for-sale
Amortized Cost		2,369
Gross Unrealized Holding Losses		3
Fair Value		2,366
Continuous Unrealized Loss Position less than 12 Months		$ 3

X

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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