Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	$ (27,369)us-gaap_NetIncomeLoss	$ (14,298)us-gaap_NetIncomeLoss
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	432us-gaap_Depreciation	167us-gaap_Depreciation
Stock based compensation expense	4,522us-gaap_ShareBasedCompensation	2,358us-gaap_ShareBasedCompensation
Amortization of debt discount and debt issuance costs	106	102
Accrual of the end of term charge on the debt	27us-gaap_AccretionExpense	33us-gaap_AccretionExpense
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(202)	(2,600)
Accounts payable	(1,389)	3,889
Accrued expenses and other current liabilities	1,723	(832)
Accrued compensation	(2,733)	(1,468)
Net cash used in operating activities	(24,883)	(12,649)
Investing activities
Purchase of fixed assets	(1,264)	(331)
Net cash used in investing activities	(1,264)	(331)
Financing activities
Payments on capital lease obligations		(16)
Proceeds from exercise of stock options	1,475	353
Net cash provided by financing activities	1,475	337
Decrease in cash and cash equivalents	(24,672)	(12,643)
Cash and cash equivalents at beginning of period	159,226	113,894
Cash and cash equivalents at end of period	134,554	101,251
Supplemental disclosures of cash flow information:
Cash paid for interest	663us-gaap_InterestPaid	463us-gaap_InterestPaid
Cash received for taxes	$ 994us-gaap_IncomeTaxesPaidNet	$ 4,389us-gaap_IncomeTaxesPaidNet

The increase (decrease) during the reporting period in the sum of accrued expenses and Other Current Liabilities.

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Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations.

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Amount of noncash expense included in interest expense to allocate debt discount and premium, and the costs to issue debt and obtain financing over the related debt instruments. Alternate captions include noncash interest expense.

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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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The amount of cash paid for interest during the period.

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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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Amount of cash inflow (outflow) from operating activities, excluding discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

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