CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (70,430)	$ (83,118)	$ (64,544)
Non-cash adjustments reconciling net loss to operating cash flows
Depreciation of property and equipment	184	180	212
Amortization of premium on investments	(266)	7	337
Share-based compensation expense	6,786	10,624	10,254
Changes in operating assets and liabilities
Other current assets	(3,409)	254	279
Other long-term assets	3,606	(1,259)	(1,675)
Accounts payable and accrued liabilities	(1,341)	5,196	4,370
Other current and long term liabilities	164	99	53
Cash flows used in operating activities	(64,706)	(68,017)	(50,714)
Investing activities:
Purchases of short-term investments	(100,558)	(70,269)	(104,954)
Sales and maturities of short-term investments	91,988	108,720	54,530
Purchases of property and equipment	(81)	(196)	(329)
Cash flows provided by (used in) investing activities	(8,651)	38,255	(50,753)
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	153,530	0	143,293
Proceeds from exercise of common stock options and warrants	5,003	2,355	552
Proceeds from issuance under employee stock purchase plan	144	297	522
Cash flows provided by financing activities	158,677	2,652	144,367
Increase (decrease) in cash and cash equivalents	85,320	(27,110)	42,900
Cash and cash equivalents, beginning of year	22,383	49,493	6,593
Cash and cash equivalents, end of year	107,703	22,383	49,493
Net exercise of warrants	$ 0	$ 0	$ 0