Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
ASSETS
Cash and cash equivalents	$ 157,968	$ 71,301
Short-term investments	84,998	224,939
Receivable from collaboration partner	67,584	35,458
Restricted cash		343
Prepaid expenses and other current assets	17,341	12,175
Total current assets	327,891	344,216
Property and equipment, net	17,097	13,262
Restricted cash, net of current	9,899	8,843
Other non-current assets	6,155	5,545
Total assets	361,042	371,866
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	5,474	2,073
Accrued expenses and other current liabilities	61,755	48,951
Deferred revenue	16,931	33,862
Total current liabilities	84,160	84,886
Convertible Notes, net of unamortized discount of $53,577 and $62,743 at September 30, 2013 and December 31, 2012, respectively	205,173	196,007
Deferred revenue, net of current	4,233	8,465
Other non-current liabilities	7,140	8,863
Total liabilities	300,706	298,221
Commitments and contingencies (Note 11)
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; 75,344,784 and 74,774,939 shares issued and outstanding at September 30, 2013 and December 31, 2012, respectively	754	748
Additional paid-in capital	396,493	364,412
Accumulated other comprehensive income	18	33
Accumulated deficit	(336,929)	(291,548)
Total stockholders' equity	60,336	73,645
Total liabilities and stockholders' equity	$ 361,042	$ 371,866