INVESTMENTS (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale
Amortized Cost	$ 372,582	$ 223,144
Gross Unrealized Holding Gains	11
Gross Unrealized Holding Losses	103	196
Fair Value	372,490	222,948
Continuous Unrealized Loss Position less than 12 Months	103	196
Short-term \| Commercial paper
Available-for-sale
Amortized Cost	73,037	40,382
Gross Unrealized Holding Gains	3
Fair Value	73,040	40,382
Short-term \| Certificates of deposit
Available-for-sale
Amortized Cost	37,210
Fair Value	37,210
Short-term \| Municipal securities
Available-for-sale
Amortized Cost	152,287	140,379
Gross Unrealized Holding Gains	8
Gross Unrealized Holding Losses	42	181
Fair Value	152,253	140,198
Continuous Unrealized Loss Position less than 12 Months	42	181
Short-term \| U.S. government agency securities
Available-for-sale
Amortized Cost	28,465	26,057
Gross Unrealized Holding Losses	22	6
Fair Value	28,443	26,051
Continuous Unrealized Loss Position less than 12 Months	22	6
Short-term \| Variable rate demand notes
Available-for-sale
Amortized Cost	32,905	13,923
Fair Value	32,905	13,923
Long-term \| Municipal securities
Available-for-sale
Amortized Cost	32,704	2,403
Gross Unrealized Holding Losses	17	9
Fair Value	32,687	2,394
Continuous Unrealized Loss Position less than 12 Months	17	$ 9
Long-term \| U.S. government agency securities
Available-for-sale
Amortized Cost	15,974
Gross Unrealized Holding Losses	22
Fair Value	15,952
Continuous Unrealized Loss Position less than 12 Months	$ 22

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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