Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
ASSETS
Cash and cash equivalents	$ 290,036	$ 228,788
Receivable from collaboration partner	99,938	107,210
Prepaid expenses and other current assets	25,656	17,981
Total current assets	415,630	353,979
Property and equipment, net	39,680	17,035
Restricted cash	11,972	9,899
Other non-current assets	15,965	11,737
Total assets	483,247	392,650
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable, accrued expenses and other current liabilities	78,195	78,758
Deferred revenue	8,466	16,931
Current portion of build-to-suit lease obligation	645
Total current liabilities	87,306	95,689
Convertible Notes, net of unamortized discount of $43,725 and $50,336 at June 30, 2014 and December 31, 2013, respectively	215,022	208,414
Build-to-suit lease obligation, excluding current portion	16,605
Other non-current liabilities	20,982	11,600
Total liabilities	339,915	315,703
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	0	0
Common stock, $0.01 par value per share; 170,000,000 shares authorized; 76,703,796 and 75,803,020 shares issued and outstanding at June 30, 2014 and December 31, 2013, respectively	767	758
Additional paid-in capital	442,472	410,350
Accumulated deficit	(299,907)	(334,161)
Total stockholders' equity	143,332	76,947
Total liabilities and stockholders' equity	$ 483,247	$ 392,650