CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018
Operating activities:
Net loss		$ (140,880)	$ (70,146)
Non-cash adjustments reconciling net loss to operating cash flows:
Depreciation of property and equipment		156	131
Accretion of discount on investments		(2,818)	(757)
Share-based compensation expense		38,826	11,592
Changes in operating assets and liabilities:
Other current assets		(6,566)	1,437
Other long-term assets		(3,873)	(204)
Accounts payable, accrued liabilities, deferred revenue and other liabilities		9,331	8,646
Cash flows used in operating activities		(105,824)	(49,301)
Investing activities:
Purchases of short-term investments		(458,115)	(155,775)
Sales and maturities of short-term investments		289,134	73,729
Purchases of property, plant, and equipment		(220)	(36)
Cash flows used in investing activities		(169,201)	(82,082)
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	[1]	327,810	130,663
Proceeds from exercise of common stock options		5,372	9,525
Proceeds from disgorgement of stockholders' short-swing profits		1,050	0
Proceeds from stock issuances under ESPP		323	202
Cash flows provided by financing activities		334,555	140,390
Increase in cash, cash equivalents and restricted cash		59,530	9,007
Cash, cash equivalents and restricted cash, beginning of period		32,694	107,703
Cash, cash equivalents and restricted cash, end of period		92,224	116,710
Reconciliation of cash, cash equivalents and restricted cash, end of period:
Total cash, cash equivalents and restricted cash		$ 32,694	$ 107,703

[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.