Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
ASSETS
Cash and cash equivalents	$ 71,301		$ 70,136
Short-term investments	224,939		74,996
Receivables from collaboration partners	35,458		12,545
Restricted cash	343		500
Prepaid expenses and other current assets	12,175		10,012
Total current assets	344,216		168,189
Property and equipment, net	13,262		720
Restricted cash, net of current	8,843		5,489
Other non-current assets	5,545		719
Total assets	371,866		175,117
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	2,073		5,588
Accrued expenses and other current liabilities	48,951		24,284
Deferred revenue	33,862		59,762
Total current liabilities	84,886		89,634
Convertible Notes, net of unamortized discount of $62,743 at December 31, 2012	196,007
Deferred revenue, net of current	8,465		83,509
Other non-current liabilities	8,863		653
Total liabilities	298,221		173,796
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; 74,774,939 and 71,463,676 shares issued and outstanding at December 31, 2012 and 2011, respectively	748		714
Additional paid-in capital	364,412		250,885
Accumulated other comprehensive income	33		13
Accumulated deficit	(291,548)		(250,291)
Total stockholders' equity	73,645	[1]	1,321	[1]
Total liabilities and stockholders' equity	$ 371,866		$ 175,117

[1] All share, par, and additional paid-in capital amounts have been retroactively adjusted to reflect the Company's September 21, 2012 two-for-one forward stock split. This stock split resulted in the issuance of approximately 37.0 million shares of the Company's common stock.