Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 399,146	$ 256,861
Other current assets	2,164	1,735
Total current assets	401,310	258,596
Property and equipment, net	1,078	1,022
Other assets	4,143	4,284
Total assets	406,531	263,902
Current liabilities:
Accounts payable	5,700	6,089
Accrued expenses	18,156	16,750
Other current liabilities	3,052	1,526
Total current liabilities	26,908	24,365
Convertible notes, net	117,507	115,481
Total liabilities	144,415	139,846
Commitments and contingencies (Note 8)
Stockholders' equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized at both December 31, 2014 and March 31, 2015; no shares issued or outstanding at both December 31, 2014 and March 31, 2015
Common stock, $0.0001 par value; 100,000,000 shares authorized at both December 31, 2014 and March 31, 2015; 36,110,082 and 40,000,977 shares issued and outstanding at December 31, 2014 and March 31, 2015, respectively	4	4
Additional paid-in capital	661,128	474,562
Accumulated deficit	(399,016)	(350,510)
Total stockholders' equity	262,116	124,056
Total liabilities and stockholders' equity	$ 406,531	$ 263,902