SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Net loss and net loss available to common stockholders	$ (18,454)	$ (15,479)	$ (14,279)	$ (14,288)	$ (6,934)	$ (10,895)	$ (19,283)	$ (8,230)	$ (28,567)	$ (44,046)	$ (62,500)	$ (45,343)	$ (124,742)
Weighted average shares outstanding, basic and diluted (in shares)											20,732	18,993	13,083
Net loss per share, basic and diluted (in dollars per share)	$ (0.82)	$ (0.75)	$ (0.69)	$ (0.70)	$ (0.35)	$ (0.55)	$ (0.97)	$ (0.51)			$ (3.01)	$ (2.39)	$ (9.54)
Anti-dilutive securities excluded from diluted net loss per share calculation for stock-based compensation plans (in shares)											1,509	1,076	1,350