Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 71,560	$ 91,297
Marketable securities	95,209	36,679
Unbilled collaboration receivable	476
Prepaid expenses and other current assets	2,502	922
Deferred tax assets, net		1,246
Total current assets	169,747	130,144
Marketable securities	27,125
Property and equipment, net	3,758	3,565
Restricted cash	571	571
Deferred tax assets, net		2,706
Other assets	4	22
Total assets	201,205	137,008
Liabilities, convertible preferred stock, and stockholders' equity (deficit)
Preferred stock
Current liabilities:
Accounts payable	3,678	3,308
Accrued expenses	6,586	1,708
Income taxes payable	1,462	4,875
Deferred revenue	25,072	25,072
Deferred rent	123	85
Restricted stock liability	9	65
Total current liabilities	36,930	35,113
Deferred revenue, net of current portion	32,567	57,639
Deferred rent, net of current portion	220	343
Restricted stock liability, net of current portion	6	15
Commitments and contingencies (Note 6)
Stockholders' equity (deficit):
Preferred stock
Common stock, $0.001 par value; 125,000,000 and 78,300,000 shares authorized at December 31, 2013 and 2012, respectively, 31,202,542 and 3,616,101 shares issued and outstanding at December 31, 2013 and 2012, respectively	31	3
Additional paid-in capital	244,881	2,012
Accumulated other comprehensive income (loss)	14	(2)
Accumulated deficit	(113,444)	(74,037)
Total stockholders' equity (deficit)	131,482	(72,024)
Total liabilities, convertible preferred stock, and stockholders' equity (deficit)	201,205	137,008
Series A Convertible Preferred Stock [Member]
Liabilities, convertible preferred stock, and stockholders' equity (deficit)
Preferred stock		32,940
Stockholders' equity (deficit):
Preferred stock		32,940
Series B Convertible Preferred Stock [Member]
Liabilities, convertible preferred stock, and stockholders' equity (deficit)
Preferred stock		5,681
Stockholders' equity (deficit):
Preferred stock		5,681
Series C Convertible Preferred Stock [Member]
Liabilities, convertible preferred stock, and stockholders' equity (deficit)
Preferred stock		77,301
Stockholders' equity (deficit):
Preferred stock		$ 77,301