Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash flows from operating activities
Net loss	$ (37,948)	$ (20,849)	$ (22,765)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	5,711	1,930	1,598
Amortization of premiums on investment securities	1,528	(133)	105
Depreciation and amortization	79	109	285
Non-cash charge resulting from lease termination			806
Gain on sales of property and equipment	(10)	(252)
Changes in operating assets and liabilities:
Prepaid expenses, receivables and other current assets	(1,989)	320	(131)
Other assets	(179)	14	106
Accounts payable	(1,003)	(585)	(29)
Accrued expenses	2,413	635	277
Deferred revenue	(379)	(2,822)	(57)
Long-term liabilities	8		(92)
Net cash used in operating activities	(31,769)	(21,633)	(19,897)
Cash flows from investing activities
Purchases of investment securities	(211,585)	(56,728)	(48,066)
Maturities of investment securities	86,087	30,948	54,049
Purchases of property and equipment	(618)		(3)
Proceeds from sales of property and equipment	12	252
Net cash (used in) provided by investing activities	(126,104)	(25,528)	5,980
Cash flows from financing activities
Proceeds from issuances of equity securities, net of issuance costs	111,682	98,204	14,022
Repayments of long-term debt		(32)	(78)
Net cash provided by financing activities	111,682	98,172	13,944
Effect of exchange rate changes on cash	(1)	(1)	13
Net (decrease) increase in cash and cash equivalents	(46,192)	51,010	40
Cash and cash equivalents
Beginning of year	57,899	6,889	6,849
End of year	11,707	57,899	6,889
Supplemental schedule of noncash investing activities
Unrealized gain (loss) on investment securities	$ 45	$ (4)	$ 9