Stock-based Compensation - Narrative (Details) - USD ($)		1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 13, 2019	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total grant date fair value for vested options						$ 3,500,000	$ 2,700,000
Stock-based compensation expense			$ 21,000,000.0	$ 22,911,000	$ 24,436,000	$ 25,012,000	26,285,000
Shares of common stock reserved (in shares)		63,320,000	63,320,000	63,320,000		63,320,000		204,278,000
Common Stock | 2010 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for issuance (in shares)								75,008,088
Number of shares available for issuance (in shares)		0	0	0		0		13,355,967
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	66,661,953		66,661,953
Number of shares available for issuance (in shares)		29,386,922	29,386,922	29,386,922		29,386,922
Number of new shares authorized for issuance (in shares)		29,335,000	29,335,000	29,335,000		29,335,000
Number of additional shares authorized for issuance (in shares)		37,326,953	37,326,953	37,326,953		37,326,953
Number of shares granted (in shares)						0
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%
Class B common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense					23,300,000		$ 23,300,000
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						4 years
Weighted-average grant date fair value for options granted (in dollars per share)						$ 4.10	$ 1.31
Unvested options exercisable (in shares)		17,118,432	17,118,432	17,118,432		17,118,432		20,697,847
Options unrecognized stock-based compensation expense		$ 18,400,000	$ 18,400,000	$ 18,400,000		$ 18,400,000		$ 15,500,000
Weighted-average remaining vesting period						2 years 9 months 18 days		3 years 9 months 18 days
Liability for early exercise of stock options		$ 14,600,000	$ 14,600,000	$ 14,600,000		$ 14,600,000		$ 14,300,000
Number of unvested shares expected to be repurchased (in shares)						6,563,716		6,737,971
Shares of common stock reserved (in shares)		21,878,000	21,878,000	21,878,000		21,878,000		25,087,000
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%
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted (in shares)						6,465,000
Vesting period						4 years
Weighted-average remaining vesting period						2 years		0 years
Vesting percentage						25.00%
Total grant date fair value for vested shares						$ 2,700,000	$ 0
Stock-based compensation expense				$ 15,400,000	$ 0	15,400,000	$ 0
Unrecognized stock-based compensation expense		$ 45,100,000	$ 45,100,000	$ 45,100,000		$ 45,100,000		$ 0
Shares of common stock reserved (in shares)		6,185,000	6,185,000	6,185,000		6,185,000		0
Restricted Stock Units (RSUs) | Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						1 year
Restricted Stock Units (RSUs) | Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						3 years
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved (in shares)		5,870,000	5,870,000	5,870,000		5,870,000		0
ESPP | 2019 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 1,500,000	$ 1,500,000	$ 1,500,000		$ 1,500,000		$ 0
Maximum ownership percentage threshold for participation	5.00%
Maximum contribution percentage per employee	10.00%
ESPP | Class A common stock | 2019 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved (in shares)	5,870,000
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)		0
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%
Initial Public Offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 21,000,000.0