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								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Net loss and net loss available to common stockholders	$ (6,934)	$ (10,895)	$ (19,283)	$ (8,230)	$ (13,110)	$ (91,297)	$ (10,693)	$ (9,644)	$ (45,343)	$ (124,742)	$ (48,443)
Weighted average shares outstanding, basic and diluted (in shares)									18,993	13,083	12,975
Net loss per share, basic and diluted (in dollars per share)	$ (0.35)	$ (0.55)	$ (0.97)	$ (0.51)	$ (1.01)	$ (7.01)	$ (0.82)	$ (0.75)	$ (2.39)	$ (9.54)	$ (3.73)
Anti-dilutive securities excluded from diluted net loss per share calculation for stock-based compensation plans (in shares)									1,076	1,350	1,664