SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NET LOSS PER COMMON SHARE (Details) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	27,317	19,944	12,106
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)	5,141	6,101	6,276
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)	153	210	64
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)	4,161	3,734	2,597
Convertible notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted earnings per common share (in shares)	17,862	9,899	3,169