Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
ASSETS
Cash and cash equivalents	$ 228,788	$ 71,301
Short-term investments		224,939
Receivable from collaboration partner	107,210	35,458
Restricted cash		343
Prepaid expenses and other current assets	17,981	12,175
Total current assets	353,979	344,216
Property and equipment, net	17,035	13,262
Restricted cash, net of current	9,899	8,843
Other non-current assets	11,737	5,545
Total assets	392,650	371,866
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable, accrued expenses and other current liabilities	78,758	51,024
Deferred revenue	16,931	33,862
Total current liabilities	95,689	84,886
Convertible Notes, net of unamortized discount of $50,336 and $62,743 at December 31, 2013 and 2012, respectively	208,414	196,007
Deferred revenue, net of current		8,465
Other non-current liabilities	11,600	8,863
Total liabilities	315,703	298,221
Commitments and contingencies (Note 12)
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; 75,803,020 and 74,774,939 shares issued and outstanding at December 31, 2013 and 2012, respectively	758	748
Additional paid-in capital	410,350	364,412
Accumulated other comprehensive income		33
Accumulated deficit	(334,161)	(291,548)
Total stockholders' equity	76,947	73,645
Total liabilities and stockholders' equity	$ 392,650	$ 371,866