SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NET LOSS PER COMMON SHARE (Details) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	25,291	27,317	19,944
Employee stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	4,195	5,141	6,101
Employee stock purchase plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	121	153	210
Restricted stock and restricted stock units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	3,118	4,161	3,734
Convertible notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	17,857	17,862	9,899