CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
ASSETS
Cash and cash equivalents	$ 100,987	$ 71,301
Short-term investments	144,956	224,939
Receivable from collaboration partner	64,319	35,458
Restricted cash	343	343
Prepaid expenses and other current assets	14,711	12,175
Total current assets	325,316	344,216
Property and equipment, net	16,324	13,262
Restricted cash, net of current	9,899	8,843
Other non-current assets	6,369	5,545
Total assets	357,908	371,866
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	3,944	2,073
Accrued expenses and other current liabilities	57,485	48,951
Deferred revenue	25,396	33,862
Total current liabilities	86,825	84,886
Convertible Notes, net of unamortized discount of $56,774 and $62,743 at June 30, 2013 and December 31, 2012, respectively	201,976	196,007
Deferred revenue, net of current		8,465
Other non-current liabilities	7,475	8,863
Total liabilities	296,276	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; 75,095,615 and 74,774,939 shares issued and outstanding at June 30, 2013 and December 31, 2012, respectively	751	748
Additional paid-in capital	384,485	364,412
Accumulated other comprehensive income	12	33
Accumulated deficit	(323,616)	(291,548)
Total stockholders' equity	61,632	73,645
Total liabilities and stockholders' equity	$ 357,908	$ 371,866