Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
ASSETS
Cash and cash equivalents	$ 241,469	$ 228,788
Receivable from collaboration partner	93,251	107,210
Prepaid expenses and other current assets	17,129	17,981
Total current assets	351,849	353,979
Property and equipment, net	18,001	17,035
Restricted cash	11,972	9,899
Other non-current assets	16,238	11,737
Total assets	398,060	392,650
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable, accrued expenses and other current liabilities	78,154	78,758
Deferred revenue	12,698	16,931
Total current liabilities	90,852	95,689
Convertible Notes, net of unamortized discount of $47,079 and $50,336 at March 31, 2014 and December 31, 2013, respectively	211,671	208,414
Other non-current liabilities	13,584	11,600
Total liabilities	316,107	315,703
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred stock, $0.01 par value per share; 1,000,000 shares authorized; no shares issued and outstanding
Common stock, $0.01 par value per share; 170,000,000 shares authorized; 76,513,737 and 75,803,020 shares issued and outstanding at March 31, 2014 and December 31, 2013, respectively	765	758
Additional paid-in capital	429,014	410,350
Accumulated deficit	(347,826)	(334,161)
Total stockholders' equity	81,953	76,947
Total liabilities and stockholders' equity	$ 398,060	$ 392,650