CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018
Operating activities:
Net loss		$ (86,607)	$ (42,578)
Non-cash adjustments reconciling net loss to operating cash flows:
Depreciation of property and equipment		99	80
Accretion of discount on investments		(1,964)	(376)
Share-based compensation expense		23,720	7,534
Changes in operating assets and liabilities:
Other current assets		(6,313)	1,510
Other long-term assets		(3,982)	(184)
Accounts payable, accrued liabilities, deferred revenue and other liabilities		2,097	4,885
Cash flows used in operating activities		(72,950)	(29,129)
Investing activities:
Purchases of short-term investments		(212,666)	(151,336)
Sales and maturities of short-term investments		171,600	40,492
Purchases of property, plant, and equipment		(69)	0
Cash flows used in investing activities		(41,135)	(110,844)
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	[1]	327,810	130,660
Proceeds from exercise of common stock options		4,300	7,371
Proceeds from disgorgement of stockholders' short-swing profits		1,050	0
Proceeds from stock issuances under employee stock purchase plan		323	202
Cash flows provided by financing activities		333,483	138,233
Increase (decrease) in cash and cash equivalents		219,398	(1,740)
Cash and cash equivalents, beginning of period		32,694	107,703
Cash and cash equivalents, end of period		$ 252,092	$ 105,963

[1] Proceeds in 2018 include warrants to purchase up to 421,650 shares of the Company's common stock at a public offering price of $38.849 per warrant, net of issuance costs.