CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities:
Net loss	$ (70,146,000)	$ (52,535,000)
Non-cash adjustments reconciling net loss to operating cash flows:
Depreciation of property and equipment	131,000	138,000
Amortization of discount or premium on short-term investments	(757,000)	(232,000)
Share-based compensation expense	11,592,000	5,468,000
Changes in operating assets and liabilities:
Other current assets	1,437,000	(2,052,000)
Other long-term assets	(204,000)	1,605,000
Accounts payable, accrued liabilities, deferred revenue and other liabilities	8,646,000	(3,332,000)
Cash flows used in operating activities	(49,301,000)	(50,940,000)
Investing activities:
Purchases of short-term investments	(155,775,000)	(95,103,000)
Sales and maturities of short-term investments	73,729,000	75,488,000
Purchases of property and equipment	(36,000)	(80,000)
Cash flows used in investing activities	(82,082,000)	(19,695,000)
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	130,663,000	66,816,000
Proceeds from exercise of common stock options and warrants	9,525,000	2,149,000
Proceeds from stock issuances under employee stock purchase plan	202,000	71,000
Cash flows provided by financing activities	140,390,000	69,036,000
Increase (decrease) in cash and cash equivalents	9,007,000	(1,599,000)
Cash and cash equivalents, beginning of period	107,703,000	22,383,000
Cash and cash equivalents, end of period	$ 116,710,000	$ 20,784,000