Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating activities
Net loss	$ (11,537)	$ (10,193)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation of property and equipment	153	142
Stock-based compensation	2,106	1,536
Noncash interest expense, net	642	409
Changes in assets and liabilities:
Accounts receivable due from related party	389	222
Prepaids and other current assets	(514)	(312)
Other assets		(127)
Accounts payable	(199)	31
Other liabilities	499	(846)
Deferred revenue from related party		(1,128)
Net cash used in operating activities	(8,461)	(10,266)
Investing activities
Purchases of property and equipment, net	(32)	(36)
Purchases of investments	(33,000)	(36,682)
Maturities of investments	41,719	48,697
Net cash provided by investing activities	8,687	11,979
Financing activities
Proceeds from exercise of stock options	1,290	1,293
Proceeds from exercise of warrants		312
Payments on equipment financing obligations	(146)	(141)
Net cash provided by financing activities	1,144	1,464
Net increase in cash and cash equivalents	1,370	3,177
Cash and cash equivalents at beginning of period	10,258	8,460
Cash and cash equivalents at end of period	11,628	11,637
Supplemental disclosures of cash flow information
Cash paid for interest	$ 3	$ 9