Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended		36 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014
Cash flows from operating activities:
Net loss	$ (7,814)	$ (5,157)	$ (52,992)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	66	47	390
Impairment of property and equipment			364
Amortization of premium on debt securities	149	108	812
Stock-based compensation expense	867	837	7,689
Issuance of common stock for technology access fee			2,500
Net loss on equity method investment	97		97
Changes in operating assets and liabilities:
Prepaid expenses and other assets	45	97	(605)
Accounts payable	1,401	73	1,555
Accrued expenses and other non-current liabilities	228	(61)	4,419
Net cash used in operating activities	(4,961)	(4,056)	(35,771)
Cash flows from investing activities:
Investment in joint venture	(1,500)		(1,500)
Purchases of property, plant and equipment		(184)	(1,568)
Maturities of short-term investments	4,714		10,384
Sales of short-term investments	4,415		4,415
Purchases of short-term investments	(16,273)		(48,945)
Increase in restricted cash			(88)
Net cash used in investing activities	(8,644)	(184)	(37,302)
Cash flows from financing activities:
Proceeds from issuance of preferred stock, net of issuance costs			41,091
Net proceeds from the issuance of common stock	51,661	32,652	88,116
Net cash provided by financing activities	51,661	32,652	129,207
Net increase in cash and cash equivalents	38,056	28,412	56,134
Cash and cash equivalents at beginning of period	18,078	14,776
Cash and cash equivalents at end of period	56,134	43,188	56,134
Supplemental disclosure of non-cash investing and financing activity
Accretion of convertible preferred stock to redemption value			(101)
Conversion of convertible preferred stock to common stock			$ 41,192