Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities
Net loss	$ (23,861)	$ (17,112)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	115	53
Amortization of debt discount and beneficial conversion	15
Amortization of debt issuance costs	16
Amortization of premiums and discounts on investments	178	106
Stock-based compensation expense	4,970	1,676
Loss related to assets held for sale		29
Loss on sale of assets		1
Changes in assets and liabilities:
Prepaids and other assets	(73)	(992)
Accounts payable	1,265	1,789
Other accrued liabilities	(502)	(732)
Net cash used in operating activities	(17,877)	(15,182)
Investing activities
Purchases of investments	(168,134)	(4,800)
Proceeds from sales/maturities of investments	39,995	6,926
Proceeds from sale of assets	6	12
Purchase of property and equipment	(21)	(638)
Net cash used in investing activities	(128,154)	1,500
Financing activities
Proceeds from issuance of common stock, net of issuance costs	189,982
Proceeds from exercise of common stock options	798	142
Proceeds from warrant issuance		78
Proceeds from debt issuance, net of issuance costs		4,869
Net cash provided by financing activities	190,780	5,089
Net increase (decrease) in cash and cash equivalents	44,749	(8,593)
Cash and cash equivalents at beginning of period	85,038	56,537
Cash and cash equivalents at end of period	$ 129,787	$ 47,944