Condensed Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Operating activities
Net loss	$ (62,076)	$ (39,741)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	742	679
Amortization of premium on investment securities	1,848	818
Stock-based compensation expense	4,726	1,923
Revaluation of convertible preferred stock warrant liability		(24)
Unrealized (gain) loss on foreign currency forward contracts	1	352
Changes in operating assets and liabilities:
Receivables from collaborators	127	(2,832)
Prepaid expenses and other current assets	(1,276)	(1,091)
Other assets	47	229
Accounts payable	2,701	(164)
Accrued compensation and employee benefits	(321)	(187)
Accrued and other liabilities	1,521	8,153
Deferred revenue	18,214	2,991
Other long-term liabilities	514	(433)
Net cash used in operating activities	(33,232)	(29,327)
Investing activities
Purchases of property and equipment	(963)	(333)
Purchases of investments	(143,493)	(98,640)
Proceeds from sales of investments	3,000	4,644
Proceeds from maturities of investments	111,016	53,936
Net cash used in investing activities	(30,440)	(40,393)
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	2,967	253
Net cash provided by financing activities	2,967	128,353
Net (decrease) increase in cash and cash equivalents	(60,705)	58,633
Cash and cash equivalents at beginning of period	117,773	53,613
Cash and cash equivalents at end of period	$ 57,068	$ 112,246