Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 321,053	$ 228,788
Receivable from collaboration partner	208,090	107,210
Prepaid expenses and other current assets	20,918	17,981
Restricted cash	410
Total current assets	550,471	353,979
Property and equipment, net	41,075	17,035
Restricted cash	11,562	9,899
Other non-current assets	15,076	11,737
Total assets	618,184	392,650
Current liabilities:
Accounts payable, accrued expenses and other current liabilities	109,060	78,758
Deferred revenue	4,233	16,931
Current portion of build-to-suit lease obligation	781
Total current liabilities	114,074	95,689
Convertible Notes, net of unamortized discount of $40,187 and $50,336 at September 30, 2014, and December 31, 2013, respectively	218,555	208,414
Build-to-suit lease obligation, excluding current portion	18,249
Other non-current liabilities	18,060	11,600
Total liabilities	368,938	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
Common stock, $0.01 par value per share; 170,000,000 shares authorized; 77,567,195 and 75,803,020 shares issued and outstanding at September 30, 2014, and December 31, 2013, respectively	776	758
Additional paid-in capital	470,384	410,350
Accumulated deficit	(221,914)	(334,161)
Total stockholders' equity	249,246	76,947
Total liabilities and stockholders' equity	$ 618,184	$ 392,650