Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Operating activities
Net loss	$ (39,859)	$ (25,923)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	475	412
Stock based compensation expense	6,410	1,518
Gain on sale of assets, net	(2)	(5)
Amortization of debt discount and debt issuance costs	256	55
Accrual of the end of term charge on the debt	126
Changes in operating assets and liabilities:
Accounts receivable		757
Prepaid expenses and other assets	(1,078)	(175)
Accounts payable	(2,399)	651
Accrued expenses and deferred rent	3,908	31
Accrued lease expense	(190)	(193)
Accrued compensation	(254)	786
Net cash used in operating activities	(32,607)	(22,086)
Investing activities
Purchase of fixed assets	(713)	(122)
Proceeds from sale of asset	2	5
Maturity of a certificate of deposit	2,153
Sales of short -term investments		17,050
Net cash provided by investing activities	1,442	16,933
Financing activities
Payments on capital lease obligations	(80)	(91)
Borrowings of long-term debt, net of issuance costs		9,726
Net proceeds from issuance of common stock	67,017	25,659
Proceeds from exercise of stock options	1,471
Net cash provided by financing activities	68,408	35,294
Increase in cash and cash equivalents	37,243	30,141
Cash and cash equivalents at beginning of period	90,782	14,848
Cash and cash equivalents at end of period	128,025	44,989
Supplemental disclosures of cash flow information:
Cash paid for interest	1,416	164
Cash paid for taxes		4
Supplemental disclosures of non-cash investing and financing activities:
Unrealized gain on investments		385
Value of warrant exercised by converting the warrant into shares of common stock ("net issuance method")	790
Value of warrant granted in connection with debt financing		$ 790