Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Operating activities
Net loss	$ (46,022)	$ (36,662)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	190	169
Amortization of debt discount and beneficial conversion	17	22
Amortization of debt issuance costs	18	24
Amortization of premiums and discounts on investments	766	396
Stock-based compensation expense	12,386	8,341
(Gain) on sale of assets		(3)
Changes in assets and liabilities:
Prepaids and other assets	(1,233)	(2,391)
Accounts payable	1,076	(80)
Other accrued liabilities	1,388	477
Net cash used in operating activities	(31,414)	(29,707)
Investing activities
Purchases of investments	(163,526)	(242,195)
Proceeds from sales/maturities of investments	165,897	76,870
Proceeds from sale of assets		9
Purchase of property and equipment	(5)	(97)
Net cash provided by (used in) investing activities	2,366	(165,413)
Financing activities
Proceeds from issuance of common stock, net of issuance costs		189,982
Proceeds from exercise of common stock options	45	1,177
Payments of long term debt	(1,193)	(253)
Net cash (used in) provided by financing activities	(1,148)	190,906
Net decrease in cash and cash equivalents	(30,196)	(4,214)
Cash and cash equivalents at beginning of period	77,336	85,038
Cash and cash equivalents at end of period	$ 47,140	$ 80,824