INVESTMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale
Amortized Cost	$ 676,129	$ 223,144
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses	831	196
Fair Value	675,299	222,948
Continuous Unrealized Loss Position less than 12 Months	831	196
Short-term \| Commercial paper
Available-for-sale
Amortized Cost	81,026	40,382
Fair Value	81,026	40,382
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	469,604	140,379
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses	740	181
Fair Value	468,865	140,198
Continuous Unrealized Loss Position less than 12 Months	740	181
Short-term \| U.S. government agency securities
Available-for-sale
Amortized Cost	61,307	26,057
Gross Unrealized Holding Losses	88	6
Fair Value	61,219	26,051
Continuous Unrealized Loss Position less than 12 Months	88	6
Short-term \| Variable rate demand notes
Available-for-sale
Amortized Cost	49,954	13,923
Fair Value	49,954	13,923
Long-term \| Municipal securities
Available-for-sale
Amortized Cost		2,403
Gross Unrealized Holding Losses		9
Fair Value		2,394
Continuous Unrealized Loss Position less than 12 Months		$ 9
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

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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