CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (514,021)	$ (384,734)	$ (255,916)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	81,122	72,565	40,055
Depreciation and amortization	31,021	24,862	20,537
Amortization/accretion of investments	(3,742)	(1,866)	1,086
Loss on disposal of property and equipment	316	891	0
Changes in assets and liabilities:
Accounts receivable	6,654	195	1,476
Accounts receivable from related party (Note 14)	562	637	(1,370)
Prepaid expenses and other assets	9,751	(5,289)	579
Right-of-use assets, operating leases	(5,664)
Accounts payable	(23,964)	15,017	(12,766)
Accrued liabilities	(3,362)	8,787	34,369
Deferred revenue	(44,119)	(65,260)	(162,321)
Deferred lease obligation	0	2,420	2,932
Operating lease liabilities	12,647
Other liabilities	(6,169)	910	(145)
Net cash used in operating activities	(458,968)	(330,865)	(331,484)
Investing activities
Purchases of marketable securities	(1,145,226)	(1,227,709)	(727,203)
Proceeds from maturities of marketable securities	993,181	783,373	800,438
Proceeds from sales of marketable securities	168,654	177,008	402,530
Purchases of property and equipment	(31,554)	(105,766)	(58,401)
Net cash (used in) provided by investing activities	(14,945)	(373,094)	417,364
Financing activities
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	0	661,111	0
Proceeds from initial public offering of common stock, net of issuance costs	0	563,026	0
Repurchases of redeemable convertible preferred stock	0	(8,182)	0
Repurchases of redeemable convertible preferred stock	0	0	(1,483)
Proceeds from issuance of common stock under equity plans	47,259	1,427	212
Proceeds from purchase of common stock under employee stock purchase plan	2,891	0	0
Charges to financing lease liabilities	971
Reimbursement of assets under financing lease obligation	0	11,635	2,724
Payments on financing lease obligation	0	(2,175)	(1,285)
Net cash provided by financing activities	51,121	1,226,842	168
Net (decrease) increase in cash, cash equivalents and restricted cash	(422,792)	522,883	86,048
Cash, cash equivalents and restricted cash, beginning of year	670,491	147,608	61,560
Cash, cash equivalents and restricted cash, end of year	247,699	670,491	147,608
Supplemental cash flow information
Income taxes paid	416	294	398
Interest in connection with financing lease obligation	0	2,998	0
Non-cash investing and financing activities
Issuance costs included in accounts payable and accrued liabilities	0	2,638	152
Purchases of property and equipment included in accounts payable and accrued liabilities	4,676	12,892	19,959
Leasehold improvements included in prepaid and other current assets	0	10,089	1,748
Lease financing obligation	$ 0	$ 10,089	$ 1,748