Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	$ (46,913)us-gaap_NetIncomeLoss	$ (30,726)us-gaap_NetIncomeLoss
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	389us-gaap_DepreciationAndAmortization	358us-gaap_DepreciationAndAmortization
Amortization of premium on investment securities	932	944
Stock-based compensation expense	5,172us-gaap_ShareBasedCompensation	2,302us-gaap_ShareBasedCompensation
Unrealized (gain) on foreign currency forward contracts		(17)
Changes in operating assets and liabilities:
Receivables from collaborators	45	145
Prepaid expenses and other current assets	(6,278)	(1,418)
Prepaid and other long-term assets	934	19
Accounts payable	(8,827)	4,115
Accrued compensation and employee benefits	(1,754)	(882)
Accrued and other liabilities	10,420	(2,806)
Deferred revenue	(2,358)	14,128
Other long-term liabilities	1,093	601
Net cash used in operating activities	(47,145)	(13,237)
Investing activities
Purchases of property and equipment	(1,063)	(395)
Purchases of investments	(90,940)us-gaap_PaymentsToAcquireInvestments	(76,555)us-gaap_PaymentsToAcquireInvestments
Proceeds from sales of investments		3,000
Proceeds from maturities of investments	101,439	31,145
Net cash provided by (used in) investing activities	9,436	(42,805)
Financing activities
Proceeds from public offering of common stock, net	108,772
Proceeds from issuance of common stock pursuant to equity award plans	4,269	1,270
Net cash provided by financing activities	113,041	1,270
Net increase (decrease) in cash and cash equivalents	75,332	(54,772)
Cash and cash equivalents at beginning of period	57,514	117,773
Cash and cash equivalents at end of period	$ 132,846	$ 63,001

Increase decrease in prepaid expense and other current assets.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of increase (decrease) in cash and cash equivalents. Cash and cash equivalents are the amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Includes effect from exchange rate changes.

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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The increase (decrease) during the reporting period in the aggregate amount of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business.

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Amount of amortization of purchase premium on nonoperating securities.

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Amount of cash inflow (outflow) of financing activities, excluding discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Amount of cash inflow (outflow) of investing activities, excluding discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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