Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (27,097)	$ (27,817)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	33	13
Amortization of premium (accretion of discount) on marketable securities, net		14
Stock-based compensation expense	1,815	785
Research and development expense settled in stock	2,717	7,255
Change in fair value of other current assets, warrant liability and other liability	505	(10,074)
Non-cash interest	127	215
Non-cash portion of loss on retirement of note payable	57
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,420)	647
Other long-term assets	(150)
Accounts payable	888	1,018
Accrued expenses and other current liabilities	3,254	8,240
Net cash used in operating activities	(19,271)	(19,704)
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of property and equipment	(29)
Purchases of marketable securities	(30,649)	(17,070)
Sales and maturities of marketable securities		4,000
Net cash used in investing activities	(30,678)	(13,070)
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Proceeds from exercise of stock options		13
Proceeds from note payable, net	20,568
Payments on note payable	(13,156)	(3,938)
Proceeds from the issuance of common stock, net	51,332
Deferred financing costs	(95)
Proceeds from the issuance of preferred stock, net	27,368	42,870
Net cash provided by financing activities	86,017	38,945
NET INCREASE IN CASH AND CASH EQUIVALENTS	36,068	6,171
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	12,303	18,653
CASH AND CASH EQUIVALENTS AT END OF PERIOD	48,371	24,824
SUPPLEMENTAL DISCLOSURES:
Cash paid for interest	555	998
NON-CASH FINANCING ACTIVITIES:
Accretion of dividends on preferred stock	9,000	7,950
Fair value of Series A-6 convertible preferred stock issued as settlement of liability	10,109
Fair value of warrants issued	1,511	1,356
Initial public offering costs included in accrued expenses and other current liabilities	1,182
Reclassification of preferred stock to common stock	$ 298,063