Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 49,663	$ 58,358
Short-term investments	79,889	117,275
Prepaid expenses and other current assets	2,084	1,967
Total current assets	131,636	177,600
Property and equipment, net	2,836	3,483
Long-term investments	45,434	33,878
Restricted cash	479	482
Total assets	180,385	215,443
Current liabilities:
Accounts payable	4,751	3,808
Accrued expenses	11,362	11,023
Deferred rent	280	206
Other current liabilities	83	95
Total current liabilities	16,476	15,132
Deferred rent, net of current portion	1,666	1,946
Total liabilities	18,142	17,078
Commitments and contingencies (Note 8)
Stockholders' equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized; none issued and outstanding
Common stock, $0.0001 par value; 100,000,000 shares authorized; 41,887,829 and 35,864,765 shares issued and outstanding at December 31, 2016 and 2015, respectively	4	4
Additional paid-in capital	528,617	455,170
Accumulated other comprehensive loss	(274)	(282)
Accumulated deficit	(366,104)	(256,527)
Total stockholders' equity	162,243	198,365
Total liabilities and stockholders' equity	$ 180,385	$ 215,443