NET INCOME (LOSS) PER SHARE (Details) - USD ($) $ / shares in Units, $ 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
Basic and diluted net loss per share
Net loss	$ (33,104)	$ (15,992)	$ (26,659)	$ (26,741)	$ (44,368)	$ (12,894)	$ (10,541)	$ (20,917)	$ (102,496)	$ (88,720)	$ (75,738)
Denominator (in shares)									62,702,404	61,296,875	60,116,530
Basic and diluted net loss per share (in dollars per share)									$ (1.63)	$ (1.45)	$ (1.26)
Potentially dilutive common shares:
Total potentially dilutive common shares (in shares)									23,549	21,302	19,421
Convertible Notes
Potentially dilutive common shares:
Total potentially dilutive common shares (in shares)									17,931	17,931	17,931
Stock options and equivalents
Potentially dilutive common shares:
Total potentially dilutive common shares (in shares)									5,618	3,371	1,490