CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (98,418)	$ (70,430)	$ (83,118)
Non-cash adjustments reconciling net loss to operating cash flows
Depreciation of property and equipment	175	184	180
(Accretion of discount) amortization of premium on investments	(1,320)	(266)	7
Share-based compensation expense	15,854	6,786	10,624
Changes in operating assets and liabilities
Other current assets	1,052	(3,409)	254
Other long-term assets	(359)	3,606	(1,259)
Accounts payable, accrued liabilities, deferred revenue and other liabilities	12,920	(1,177)	5,295
Cash flows used in operating activities	(70,096)	(64,706)	(68,017)
Investing activities:
Purchases of short-term investments	(255,795)	(100,558)	(70,269)
Sales and maturities of short-term investments	110,152	91,988	108,720
Purchases of property and equipment	(122)	(81)	(196)
Cash flows provided by (used in) investing activities	(145,765)	(8,651)	38,255
Financing activities:
Proceeds from issuance of common stock and warrants, net of issuance costs	130,663	153,530	0
Proceeds from exercise of common stock options and warrants	9,747	5,003	2,355
Proceeds from issuances under employee stock purchase plan	442	144	297
Cash flows provided by financing activities	140,852	158,677	2,652
Increase (decrease) in cash and cash equivalents	(75,009)	85,320	(27,110)
Cash and cash equivalents, beginning of year	107,703	22,383	49,493
Cash and cash equivalents, end of year	$ 32,694	$ 107,703	$ 22,383