Stock-based Compensation - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 13, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total grant date fair value for vested options				$ 5,500,000	$ 3,500,000
Issuance of common stock in connection with acquisition (in shares)				948,000
Stock-based compensation expense		$ 14,745,000	$ 22,911,000	$ 40,091,000	$ 25,012,000
Class A common stock | 2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for issuance (in shares)		66,661,953		66,661,953
Number of new shares authorized for issuance (in shares)		29,335,000		29,335,000
Number of additional shares authorized for issuance (in shares)		37,326,953		37,326,953
Number of shares available for issuance (in shares)		25,164,731		25,164,731
Class A and Class B Common Stock | 2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential increase in number of shares authorized, as a percentage of total common stock outstanding				5.00%
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value for options granted (in dollars per share)				$ 9.74	$ 4.10
Unvested options exercisable (in shares)		11,929,262		11,929,262		15,477,903
Options unrecognized stock-based compensation expense		$ 23,500,000		$ 23,500,000		$ 15,800,000
Weighted-average remaining vesting period				2 years 9 months 18 days		2 years 8 months 12 days
Liability for early exercise of stock options		$ 9,700,000		$ 9,700,000		$ 13,300,000
Number of unvested shares expected to be repurchased (in shares)				4,354,030		5,945,083
Options | 2010 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Expiration period				10 years
Options | Common stock | 2010 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of common stock, percentage of fair market value				100.00%
Options | Class A common stock | 2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative shares granted (in shares)		1,710,189		1,710,189
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Weighted-average remaining vesting period				3 years 4 months 24 days		2 years 6 months
Stock-based compensation expense		$ 10,300,000	15,400,000	$ 27,800,000	$ 15,400,000
Unrecognized stock-based compensation expense		$ 109,200,000		109,200,000		$ 53,100,000
Total grant date fair value for vested shares				$ 19,600,000	2,700,000
Restricted Stock Units (RSUs) | Class A common stock | 2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative shares granted (in shares)		3,310,238		3,310,238
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of unvested restricted stock in connection with acquisition (in shares)				841,000
Restricted stock issued in connection with acquisition, aggregate grant date fair value				$ 1,800,000	0
Stock-based compensation expense				4,200,000	0
Unrecognized stock-based compensation expense		$ 10,200,000	$ 0	$ 10,200,000	$ 0
Restricted Stock | Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				2 years
Vesting percentage				77.80%
Restricted Stock | Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				3 years
Shares available for issuance under the ESPP | 2019 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average remaining vesting period				1 month 6 days		4 months 24 days
Unrecognized stock-based compensation expense		$ 500,000		$ 500,000		$ 1,000,000.0
Maximum ownership percentage threshold for participation	5.00%
Maximum contribution percentage per employee	10.00%
Shares available for issuance under the ESPP | Class A common stock | 2019 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares allowable under the plan (in shares)	5,870,000
Purchase price of common stock, percentage of fair value	85.00%
Offering period	6 months
Purchase period	6 months
Maximum number of shares available for repurchase for each employee (in shares)	1,500
Maximum value of shares available for repurchase for each employee	$ 25,000
Number of shares repurchased (in shares)				421,300
Shares available for issuance under the ESPP | Class A and Class B Common Stock | 2019 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential increase in number of share authorized, as a percentage of total common stock outstanding	1.00%