SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Net Loss Per Share (Details) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Common shares not included in the computation of diluted net loss per share
Potentially issuable common shares not included in the computation of diluted net loss per share because they would have an anti-dilutive effect	27,546	18,707
Shares issuable upon exercise of stock options
Common shares not included in the computation of diluted net loss per share
Potentially issuable common shares not included in the computation of diluted net loss per share because they would have an anti-dilutive effect	2,575	2,387
Shares issuable upon the release of restricted stock awards
Common shares not included in the computation of diluted net loss per share
Potentially issuable common shares not included in the computation of diluted net loss per share because they would have an anti-dilutive effect	4,662	4,123
Shares issuable upon conversion of convertible notes
Common shares not included in the computation of diluted net loss per share
Potentially issuable common shares not included in the computation of diluted net loss per share because they would have an anti-dilutive effect	20,309	12,197