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Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Oct. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Apr. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term		4 years
Share-based compensation expense		$ 46,063,000	$ 18,566,000
Accrued and other liabilities, related to shares held by employees and directors that were subject to repurchase		$ 2,800,000	$ 4,600,000
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares as percentage of common shares outstanding		5.00%
Number of shares reserved for future issuance (in shares)		9,642,503	8,176,125
Issuance of common stock for exercise of stock options (in shares)		711,123	5,020,580
2018 Plan \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power by individual		10.00%
2018 Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted period		10 years
Option exercise price as percentage of fair value of common stock on grate date		100.00%
Option exercise price as percentage of fair value of common stock on grate date		110.00%
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term		4 years
Estimated weighted average grant date fair value of employee options granted (in dollars per share)		$ 18.42	$ 3.75
Unrecognized stock based compensation		$ 74,700,000	$ 42,800,000
Weighted-average period recognized		3 years 15 days	3 years 6 months
Period of recognized compensation cost		3 years 15 days	3 years 6 months
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term		4 years
Unrecognized stock based compensation		$ 43,000,000.0	$ 0
Weighted-average period recognized		1 year 11 months 23 days
Share-based compensation expense		$ 8,800,000	$ 0
Period of recognized compensation cost		1 year 11 months 23 days
Restricted stock units, weighted average grant date fair value, grants (in dollars per share)		$ 27.45
Restricted stock units, granted (in shares)		1,982,855
Restricted Stock Units \| Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, granted (in shares)			57,361
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 1,600,000	$ 400,000
Initially reserved common stock for employee purchase (in shares)	1,160,000	1,214,826
Current offering period	24 months
Purchase of common stock through payroll deductions to equal price of lower fair market value	85.00%
Percentage of eligible compensation contribution by employee	15.00%
2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 21,900,000	3,300,000
Founders Stock Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock based compensation		$ 30,900,000	$ 44,600,000
Weighted-average period recognized		2 years 3 months	3 years 3 months
Number of vested shares (in shares)		6,057,684	6,562,506
Share-based compensation expense		$ 13,700,000	$ 14,900,000
Number of shares unvested (in shares)		13,629,803	19,687,487
Period of recognized compensation cost		2 years 3 months	3 years 3 months
Restricted stock units, weighted average grant date fair value, grants (in dollars per share)		$ 2.27
Founders Stock Award \| Founders
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares unvested (in shares)				24,230,750
Early Exercised Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock for exercise of stock options (in shares)		0	5,020,580
Other long term liabilities, related to shares held by employees and directors that were subject to repurchase		$ 3,900,000	$ 6,800,000