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INVESTMENTS (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale
Amortized Cost	$ 364,747	$ 320,740
Gross Unrealized Holding Gains	252	16
Gross Unrealized Holding Losses	5	106
Fair Value	364,994	320,650
Continuous Unrealized Loss Position less than 12 Months	5	106
Short-term | Commercial paper
Available-for-sale
Amortized Cost	63,286	52,838
Fair Value	63,286	52,838
Short-term | Certificates of deposit
Available-for-sale
Amortized Cost	15,026	14,075
Fair Value	15,026	14,075
Short-term | Municipal securities
Available-for-sale
Amortized Cost	104,230	151,690
Gross Unrealized Holding Gains	80	16
Gross Unrealized Holding Losses	3	62
Fair Value	104,307	151,644
Continuous Unrealized Loss Position less than 12 Months	3	62
Short-term | U.S. government agency securities
Available-for-sale
Amortized Cost	20,430	19,943
Gross Unrealized Holding Gains	17
Gross Unrealized Holding Losses		12
Fair Value	20,447	19,931
Continuous Unrealized Loss Position less than 12 Months		12
Short-term | U.S. treasuries
Available-for-sale
Amortized Cost	131,508	78,189
Gross Unrealized Holding Gains	141
Gross Unrealized Holding Losses	2	32
Fair Value	131,647	78,157
Continuous Unrealized Loss Position less than 12 Months	2	32
Short-term | Variable rate demand notes
Available-for-sale
Amortized Cost	23,275	4,005
Fair Value	23,275	$ 4,005
Long-term | Municipal securities
Available-for-sale
Amortized Cost	6,992
Gross Unrealized Holding Gains	14
Fair Value	$ 7,006

X

- Definition

Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, including portion attributable to noncontrolling interest. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners).

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