Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities
Net loss	$ (24,231)	$ (22,006)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	53	54
Stock-based compensation expense	2,363	1,988
Changes in assets and liabilities
Prepaid expenses and other assets	(255)	(1,456)
Accounts payable	(293)	(657)
Accrued liabilities and other liabilities	1,314	1,553
Net cash used in operating activities	(21,049)	(20,524)
Cash flows from investing activities
Purchase of property and equipment	(90)	(5)
Net cash used in investing activities	(90)	(5)
Cash flows from financing activities
Proceeds from issuance of common stock in follow-on offering, net of issuance costs		80,208
Proceeds from issuance of common stock in connection with employee benefit plans	3	16
Net cash provided by financing activities	3	80,224
Net increase (decrease) in cash and cash equivalents	(21,136)	59,695
Cash and cash equivalents at beginning of period	182,069	170,566
Cash and cash equivalents at end of period	$ 160,933	$ 230,261