Condensed Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (166,988)	$ (74,978)	$ (49,784)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	258	252	236
Amortization of debt discount	11	21	29
Amortization of debt issuance costs	12	23	32
Amortization of premiums and discounts on investments	334	1,014	647
Stock-based compensation expense	18,605	15,957	12,726
Loss on sale of assets			47
Changes in assets and liabilities:
Prepaids and other assets	(1,731)	139	(1,275)
Accounts payable	15,758	3,888	(1,333)
Other accrued liabilities	2,439	5,954	519
Net cash used in operating activities	(131,302)	(47,730)	(38,156)
Investing activities
Purchases of investments	(219,577)	(197,230)	(280,559)
Proceeds from sales/maturities of investments	183,743	207,442	120,792
Proceeds from sale of assets			24
Purchase of property and equipment	(19)	(94)	(325)
Net cash (used in) provided by investing activities	(35,853)	10,118	(160,068)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	164,000		189,983
Proceeds from exercise of common stock options	1,167	45	1,177
Payments on long-term debt	(1,709)	(1,604)	(638)
Net cash provided by (used in) financing activities	163,458	(1,559)	190,522
Net decrease in cash and cash equivalents	(3,697)	(39,171)	(7,702)
Cash and cash equivalents at beginning of period	38,165	77,336	85,038
Cash and cash equivalents at end of period	34,468	$ 38,165	$ 77,336
Supplemental disclosure of cash flow information:
Offering costs not yet paid	$ 22