Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 117,773	$ 53,613
Short-term investments	150,892	77,656
Receivables from collaborations	309	662
Prepaid expenses and other current assets	3,733	2,982
Total current assets	272,707	134,913
Property and equipment, net	2,600	2,861
Long-term investments	50,371	6,115
Other assets	53	2,112
Total assets	325,731	146,001
Current liabilities:
Accounts payable	3,232	4,840
Accrued compensation and employee benefits	2,569	1,860
Accrued and other liabilities	17,796	7,399
Deferred revenue, current portion	1,958	4,042
Convertible preferred stock warrant liability		683
Total current liabilities	25,555	18,824
Deferred revenue, long-term	3,253
Other long-term liabilities	588	1,466
Total liabilities	29,396	20,290
Commitments and contingencies (Note 11)
Convertible preferred stock, $0.001 par value, 0 and 243,258,300 shares authorized at December 31, 2013 and 2012; 0 and 24,026,797 shares issued and outstanding at December 31, 2013 and 2012; redemption value of $317,280 at December 31, 2012		317,280
Stockholders’ equity (deficit):
Preferred stock, $0.001 par value, 5,000,000 shares authorized at December 31, 2013; 0 shares issued and outstanding at December 31, 2013; 0 authorized, issued and outstanding at December 31, 2012
Common stock, $0.001 par value, 100,000,000 and 300,000,000 shares authorized at December 31, 2013 and 2012; 40,915,130 and 1,385,508 shares issued and outstanding at December 31, 2013 and 2012	41	1
Additional paid-in capital	581,911	10,717
Accumulated deficit	(285,672)	(202,320)
Accumulated other comprehensive income	55	33
Total stockholders’ equity (deficit)	296,335	(191,569)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 325,731	$ 146,001