Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 164,158	$ 355,968
Marketable securities	231,436	79,672
Prepaid expenses and other current assets	5,145	3,595
Total current assets	400,739	439,235
Property and equipment, net	15,402	4,018
Long-term marketable securities	52,032	38,411
Other assets	7,873	7,499
Total assets	476,046	489,163
Current liabilities:
Accounts payable	866	1,096
Accrued liabilities	12,410	14,577
Success payment liabilities	123,830	84,920
Build-to-suit lease obligation	341
Total current liabilities	137,447	100,593
Build-to-suit lease obligation, less current portion	9,545
Other long-term liabilities		38
Commitments and Contingencies (Note 9)
Stockholders' equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized at March 31, 2015 and December 31, 2014; 0 shares issued and outstanding at March 31, 2015 and December 31, 2014
Common stock, $0.0001 par value, 495,000,000 shares authorized at March 31, 2015 and December 31, 2014; 82,831,784 and 82,073,647 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively	8	8
Additional paid-in-capital	740,310	734,895
Accumulated other comprehensive loss	(18)	(90)
Accumulated deficit	(411,246)	(346,281)
Total stockholders' equity	329,054	388,532
Total liabilities and stockholders' equity	$ 476,046	$ 489,163