Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 165,870	$ 29,226
Marketable securities	45,798	70,435
Prepaid expenses and other current assets	1,278	1,498
Total current assets	212,946	101,159
Property and equipment, net	1,942	1,847
Other assets	413	420
Total assets	215,301	103,426
Current liabilities:
Accounts payable	7,414	4,690
Accrued personnel costs	2,885	4,858
Accrued costs for services	3,100	2,103
Accrued interest	998	2,857
Notes payable, current portion	4,518	5,484
Other current liabilities	621	637
Total current liabilities	19,536	20,629
Notes payable-net of current portion	24,396	22,661
Other liabilities	699	698
Total liabilities	44,631	43,988
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.00001 par value, 5,000 shares authorized and none issued and outstanding
Common stock, $0.00001 par value, 300,000 shares authorized, 25,799 and 22,691 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively
Additional paid in capital	507,335	367,329
Accumulated other comprehensive loss	(15)	(53)
Accumulated deficit	(336,650)	(307,838)
Total stockholders' equity	170,670	59,438
Total liabilities and stockholders' equity	$ 215,301	$ 103,426