Condensed Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Operating activities
Net income (loss)	$ (39,741)	$ 36,636
Adjustments to reconcile net income (loss) to cash used in operating activities:
Depreciation and amortization	679	699
Amortization of premium on investment securities	818	578
Stock-based compensation expense	1,923	1,411
Revaluation of convertible preferred stock warrant liability	(24)	(127)
Unrealized loss on foreign currency forward contracts	352	694
Changes in operating assets and liabilities:
Receivables from collaborations	(2,832)	(1,585)
Prepaid expenses and other current assets	(1,091)	(2,244)
Other assets	229	(271)
Accounts payable	(164)	6,434
Accrued compensation and employee benefits	(187)	(1,099)
Accrued and other liabilities	8,153	454
Deferred revenue	2,991	(68,730)
Other long-term liabilities	(433)	(410)
Net cash used in operating activities	(29,327)	(27,560)
Investing activities
Purchases of property and equipment	(333)	(354)
Purchases of investments	(98,640)	(117,147)
Proceeds from sales of investments	4,644	20,209
Proceeds from maturities of investments	53,936	14,329
Net cash used in investing activities	(40,393)	(82,963)
Financing activities
Proceeds from initial public offering, net of underwriters discount	131,027
Payment of initial public offering costs	(2,927)
Proceeds from issuance of common stock, including early exercise of stock options	253	292
Net cash provided by financing activities	128,353	292
Net increase (decrease) in cash and cash equivalents	58,633	(110,231)
Cash and cash equivalents at beginning of period	53,613	170,323
Cash and cash equivalents at end of period	$ 112,246	$ 60,092