MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 164,116	$ 94,717
Gross Unrealized Gains	538	2
Gross Unrealized Losses	(5)	(268)
Estimated Fair Market Value	164,649	94,451
Investments due in 3 months or less:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,141	6,788
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(2)
Estimated Fair Market Value	4,141	6,786
Investments due in 4-12 months:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	68,989	60,123
Gross Unrealized Gains	310	0
Gross Unrealized Losses	0	(244)
Estimated Fair Market Value	69,299	59,879
Investments due in 12 months or greater:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	90,986	27,806
Gross Unrealized Gains	228	2
Gross Unrealized Losses	(5)	(22)
Estimated Fair Market Value	91,209	27,786
Commercial paper | Investments due in 3 months or less:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,241
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Market Value	3,241
Corporate securities | Investments due in 3 months or less:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	900	6,788
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(2)
Estimated Fair Market Value	900	6,786
Corporate securities | Investments due in 4-12 months:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	68,989	60,123
Gross Unrealized Gains	310	0
Gross Unrealized Losses	0	(244)
Estimated Fair Market Value	69,299	59,879
Corporate securities | Investments due in 12 months or greater:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	90,986	27,806
Gross Unrealized Gains	228	2
Gross Unrealized Losses	(5)	(22)
Estimated Fair Market Value	$ 91,209	$ 27,786