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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