Condensed Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Operating activities
Net income (loss)	$ (58,290)	$ 25,148
Adjustments to reconcile net income (loss) to cash used in operating activities:
Depreciation and amortization	1,015	1,053
Amortization of premium on investment securities	1,523	1,018
Stock-based compensation expense	3,457	2,017
Revaluation of convertible preferred stock warrant liability	(24)	(144)
Unrealized (gain) loss on foreign currency forward contracts	(97)	450
Changes in operating assets and liabilities:
Receivables from collaborations	435	(2,347)
Prepaid expenses and other current assets	(540)	(2,155)
Other assets	361	(368)
Accounts payable	(3,323)	711
Accrued compensation and employee benefits	3	(918)
Accrued and other liabilities	10,004	1,299
Deferred revenue	3,225	(69,468)
Other long-term liabilities	(655)	(620)
Net cash used in operating activities	(42,906)	(44,324)
Investing activities
Purchases of property and equipment	(582)	(362)
Purchases of investments	(152,793)	(133,073)
Proceeds from sales of investments	6,644	42,767
Proceeds from maturities of investments	68,211	18,637
Net cash used in investing activities	(78,520)	(72,031)
Financing activities
Proceeds from initial public offering, net of underwriters discount	131,026
Payment of initial public offering costs	(5,025)
Proceeds from issuance of common stock, including early exercise of stock options	522	292
Net cash provided by financing activities	126,523	292
Net increase (decrease) in cash and cash equivalents	5,097	(116,063)
Cash and cash equivalents at beginning of period	53,613	170,323
Cash and cash equivalents at end of period	$ 58,710	$ 54,350