CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Operating activities
Net loss	$ (213,256)	$ (98,418)		$ (70,430)
Non-cash adjustments reconciling net loss to operating cash flows
Depreciation of property and equipment	249	175		184
Accretion of discount on investments	(3,421)	(1,320)		(266)
Share-based compensation expense	55,537	15,854		6,786
Changes in operating assets and liabilities:
Other current assets	(5,487)	1,052		(3,409)
Other long-term assets	(4,375)	(359)		3,606
Accounts payable, accrued liabilities, deferred revenue and other liabilities	23,027	12,920		(1,177)
Cash flows used in operating activities	(147,726)	(70,096)		(64,706)
Investing activities:
Purchases of short-term investments	(530,228)	(255,795)		(100,558)
Sales and maturities of short-term investments	355,640	110,152		91,988
Purchases of property and equipment	(1,552)	(122)		(81)
Cash flows used in investing activities	(176,140)	(145,765)		(8,651)
Financing activities:
Proceeds from issuance of common stock and warrants, net of issuance costs	327,830	130,663	[1]	153,530	[2]
Proceeds from exercise of common stock options and warrants	8,473	9,747		5,003
Proceeds from disgorgement of stockholders' short-swing profits	1,050	0		0
Proceeds from issuances under employee stock purchase plan	675	442		144
Cash flows provided by financing activities	338,028	140,852		158,677
Increase (decrease) in cash, cash equivalents and restricted cash	14,162	(75,009)		85,320
Cash, cash equivalents and restricted cash, beginning of year	32,694	107,703		22,383
Cash, cash equivalents and restricted cash, end of year	46,856	32,694		107,703
Reconciliation of cash, cash equivalents and restricted cash, end of period:
Total cash, cash equivalents and restricted cash	$ 32,694	$ 32,694		$ 22,383

[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.

[2] Proceeds in 2017 include warrants to purchase up to 4,137,999 shares of the Company's common stock at a public offering price of $12.999 per warrant, net of issuance costs, and warrants to purchase up to 7,258,263 shares of the Company's common stock at a public offering price of $5.599 per warrant, net of issuance costs.