CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities:
Net loss	$ (42,578)	$ (36,185)
Non-cash adjustments reconciling net loss to operating cash flows:
Depreciation	80	91
Amortization of premium on investments	(376)	(178)
Share-based compensation expense	7,534	3,989
Changes in operating assets and liabilities:
Other current assets	1,510	(2,054)
Other long-term assets	184	(1,605)
Accounts payable, accrued liabilities, deferred revenue and other liabilities	4,885	(3,117)
Cash flows used in operating activities	(29,129)	(35,849)
Investing activities:
Purchases of short-term investments	(151,336)	(86,424)
Disposal and maturities of short-term investments	40,492	53,038
Purchases of property and equipment	0	80
Cash flows used in investing activities	(110,844)	(33,466)
Financing activities:
Proceeds from sale of common stock, net	130,660	66,816
Proceeds from exercise of common stock options	7,371	0
Proceeds from stock issuances under employee stock purchase plan	202	71
Cash flows provided by financing activities	138,233	66,887
Decrease in cash and cash equivalents	(1,740)	(2,428)
Cash and cash equivalents, beginning of period	107,703	22,383
Cash and cash equivalents, end of period	$ 105,963	$ 19,955