Condensed Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating activities
Net loss	$ (30,726)	$ (18,142)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	358	336
Amortization of premium on investment securities	944	445
Stock-based compensation expense	2,302	713
Revaluation of convertible preferred stock warrant liability		(35)
Unrealized (gain) loss on foreign currency forward contracts	(17)	534
Changes in operating assets and liabilities:
Receivables from collaborations	145	(296)
Prepaid expenses and other current assets	(1,418)	417
Other assets	19	(971)
Accounts payable	4,115	2,068
Accrued compensation and employee benefits	(882)	(791)
Accrued and other liabilities	(2,806)	1,879
Deferred revenue	14,128	1,892
Other long-term liabilities	601	(209)
Net cash used in operating activities	(13,237)	(12,160)
Investing activities
Purchases of property and equipment	(395)	(293)
Purchases of investments	(76,555)	(11,867)
Proceeds from sales of investments	3,000	2,500
Proceeds from maturities of investments	31,145	26,009
Net cash (used in) provided by investing activities	(42,805)	16,349
Financing activities
Proceeds from issuance of common stock, including early exercise of stock options	1,270	164
Net cash provided by financing activities	1,270	164
Net (decrease) increase in cash and cash equivalents	(54,772)	4,353
Cash and cash equivalents at beginning of year	117,773	53,613
Cash and cash equivalents at end of year	$ 63,001	$ 57,966