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INVESTMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale
Amortized Cost	$ 320,740	$ 676,129
Gross Unrealized Holding Gains	16	1
Gross Unrealized Holding Losses	106	831
Fair Value	320,650	675,299
Continuous Unrealized Loss Position less than 12 Months	106	831
Short-term | Commercial paper
Available-for-sale
Amortized Cost	52,838	81,026
Fair Value	52,838	81,026
Short-term | Certificates of deposit
Available-for-sale
Amortized Cost	14,075	11,869
Fair Value	14,075	11,869
Short-term | Municipal securities
Available-for-sale
Amortized Cost	151,690	469,604
Gross Unrealized Holding Gains	16	1
Gross Unrealized Holding Losses	62	740
Fair Value	151,644	468,865
Continuous Unrealized Loss Position less than 12 Months	62	740
Short-term | U.S. government agency securities
Available-for-sale
Amortized Cost	19,943	61,307
Gross Unrealized Holding Losses	12	88
Fair Value	19,931	61,219
Continuous Unrealized Loss Position less than 12 Months	12	88
Short-term | U.S. Treasuries
Available-for-sale
Amortized Cost	78,189
Gross Unrealized Holding Losses	32
Fair Value	78,157
Continuous Unrealized Loss Position less than 12 Months	32
Short-term | Variable rate demand notes
Available-for-sale
Amortized Cost	4,005	49,954
Fair Value	$ 4,005	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

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

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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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