Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (68,606)	$ (46,447)	$ (42,831)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation expense	6,093	4,301	4,102
Share-based compensation expense	13,124	11,149	7,009
Intangible assets amortization	1,698	1,699	1,694
Investment amortization	(4,463)	(1,214)	342
Asset impairment		17	193
Loss (gain) on equipment disposals	316	(40)	125
Fair value adjustment of convertible preferred stock warrant	2,266	(6)	(23)
Other	43	5	6
Changes in operating assets and liabilities:
Accounts receivable, net	(7,817)	775	(2,427)
Inventory	(1,231)	(3,046)	(2,697)
Prepaid expenses and other current assets	(8,576)	(318)	(327)
Accounts payable and accrued liabilities	6,149	2,185	(1,517)
Deferred rent	78	(488)	(1,058)
Deferred revenue	266,927	(649)	2,527
Other	(597)	(182)	24
Net cash provided by (used in) operating activities	205,404	(32,259)	(34,858)
Investing activities
Purchases of property and equipment	(11,200)	(6,318)	(2,421)
Proceeds from sales of equipment		19	207
Purchases of intangible assets			85
Purchases of marketable securities	(884,217)	(146,503)	(125,182)
Proceeds from sales and maturities of marketable securities	413,720	153,538	163,913
Net cash (used in) provided by investing activities	(481,697)	736	36,432
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs			49,827
Proceeds from exercise of stock options	4,055	1,268	222
Proceeds from initial public offering, net of underwriting discounts and commissions	320,850
Payment of deferred initial public offering costs	(4,986)
Proceeds from issuance of common stock upon the exercise of a common stock warrant	9
Other	(12)	(20)	(15)
Net cash provided by financing activities	319,916	1,248	50,034
Net increase (decrease) in cash, cash equivalents and restricted cash	43,623	(30,275)	51,608
Cash, cash equivalents and restricted cash at beginning of year	55,091	85,366	33,758
Cash, cash equivalents and restricted cash at end of year	98,714	55,091	85,366
Noncash investing and financing activities
Purchases of equipment included in accounts payable and accrued liabilities	773	832	$ 41
Landlord-funded leasehold improvements		$ 2,419
Noncash additions to property through lease financing arrangements	36,607
Conversion of convertible preferred stock to common stock upon closing of initial public offering	561,931
Conversion of convertible preferred stock warrant to common stock warrant upon closing of initial public offering	$ 2,602