STOCKHOLDERS' EQUITY - JonesTrading Agreements (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		12 Months Ended
	Apr. 07, 2019	Aug. 09, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Class of Stock [Line Items]
Proceeds from security token offering, net				$ 92,900	$ 104,800
Proceeds from sale of common stock, net of offering costs			$ 30,957	0
Proceeds from sale of common stock included in accounts receivable			$ 8,957	0
At-The-Market Agreement | JonesTrading Institutional Services LLC
Class of Stock [Line Items]
Sales commission fee percentage		2.00%
Value of common stock available for sale under sales agreement		$ 150,000
Term of sales agreement		3 years
Aggregate offering price			5,097,410
Proceeds from sale of common stock			$ 134,500
Offering costs associated with the sale of common stock			$ 3,100
Sale of stock (in usd per share)			$ 18.25
Subsequent Event | At-The-Market Agreement | JonesTrading Institutional Services LLC
Class of Stock [Line Items]
Aggregate offering price	745,737
Additional paid-in capital
Class of Stock [Line Items]
Proceeds from sale of common stock, net of offering costs			$ 39,914	$ 0
Additional paid-in capital | Subsequent Event
Class of Stock [Line Items]
Proceeds from sale of common stock, net of offering costs	$ 12,400