Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 198,644	$ 125,445
Other current assets	889	1,175
Total current assets	199,533	126,620
Property and equipment, net	490	219
Other assets	533	541
Total assets	200,556	127,380
Current liabilities:
Accounts payable	2,105	3,170
Accrued expenses	9,501	8,545
Other current liabilities	13	3
Total current liabilities	11,619	11,718
Other non-current liabilities	13
Commitments and contingencies (Note 7)
Stockholders' equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized at both December 31, 2012 and March 31, 2013; no shares issued and outstanding at both December 31, 2012 and March 31, 2012
Common stock, $0.0001 par value; 100,000,000 shares authorized at both December 31, 2012 and March 31, 2013; 27,136,329 and 32,578,753 shares issued and outstanding at December 31, 2012 and March 31, 2013, respectively	3	3
Additional paid-in capital	294,926	202,795
Deficit accumulated during the development stage	(106,005)	(87,136)
Total stockholders' equity	188,924	115,662
Total liabilities and stockholders' equity	$ 200,556	$ 127,380