Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 57,514	$ 117,773
Short-term investments	251,759	150,892
Receivables from collaborations	57	309
Prepaid expenses and other current assets	5,747	3,733
Total current assets	315,077	272,707
Property and equipment, net	2,776	2,600
Long-term investments	83,030	50,371
Prepaid and other long-term assets	15,612	53
Total assets	416,495	325,731
Current liabilities:
Accounts payable	14,084	3,232
Accrued compensation and employee benefits	3,512	2,569
Accrued and other liabilities	13,966	17,796
Deferred revenue, current portion	9,569	1,958
Total current liabilities	41,131	25,555
Deferred revenue, long-term	27,016	3,253
Other long-term liabilities	546	588
Total liabilities	68,693	29,396
Stockholders’ equity:
Preferred stock, $0.001 par value, 5,000,000 shares authorized at December 31, 2014 and 2013; no shares issued and outstanding at December 31, 2014 and 2013;
Common stock, $0.001 par value, 100,000,000 shares authorized at December 31, 2014 and 2013; 48,766,806 shares and 40,915,130 shares issued and outstanding at December 31, 2014 and 2013, respectively	49	41
Additional paid-in capital	770,789	581,911
Accumulated deficit	(422,797)	(285,672)
Accumulated other comprehensive (loss) income	(239)	55
Total stockholders’ equity	347,802	296,335
Total liabilities and stockholders’ equity	$ 416,495	$ 325,731