Stock-based Compensation - Additional Information (Detail) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended			4 Months Ended	9 Months Ended		12 Months Ended
	Feb. 28, 2021 USD ($) $ / shares	Feb. 28, 2021 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) period tranche $ / shares shares	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2021	Sep. 30, 2021 USD ($) period tranche $ / shares shares	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2017	Dec. 31, 2004	Mar. 31, 2020 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance (in shares)			156,305,000				156,305,000		454,958,000
Share based payment arrangement, unvested award options, cost not yet recognized, amount | $			$ 180,100				$ 180,100
Share based compensation by share based payment arrangement options granted (in shares)			0				0
Allocated share based compensation | $			$ 89,319		$ 13,296		$ 221,722	$ 62,962
Tender Offer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation | $									$ 35,200
Tender Offer | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares offered by existing shareholders to buy shares from employees and former employees (in shares)												31,100,000
Tender Offer | Series G Redeemable Convertible Preferred Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares offered by existing shareholders to buy shares from employees and former employees (in shares)												24,000,000
Two Thousand and Twenty Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options to be granted price as a percentage of fair value									110.00%
Percentage of voting stock eligible for options									10.00%
Two Thousand and Twenty Equity Incentive Plan | Common Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance (in shares)			60,000,000				60,000,000
Two Thousand and Twenty Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation | $			$ 3,100				$ 6,500
Two Thousand and Twenty Employee Stock Purchase Plan | Common Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options to be granted price as a percentage of fair value									85.00%
Common stock shares reserved for future issuance (in shares)									6,000,000
Offering period, employee stock purchase plan							24 months
Number of purchase periods, employee stock purchase plan | period			4				4
Purchase period, employee stock purchase plan							6 months
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance (in shares)			69,847,000				69,847,000		98,502,000
Share based payment arrangement, unvested award, period for recognition							2 years 10 months 24 days
Options | Two Thousand and Four Incentive Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of shares available for issuance (in shares)			0				0
Options | Two Thousand and Seventeen Amended and Restated Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of voting stock eligible for options										10.00%
Share based compensation by share based payment arrangement number of shares available for issuance (in shares)			0				0
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance (in shares)			10,844,000				10,844,000		3,061,000
Share based payment arrangement, unvested award, period for recognition							3 years 4 months 24 days
Unrecognized compensation, equity instruments other than options | $			$ 595,500				$ 595,500
Allocated share based compensation | $				$ 21,300
Service period						4 years
Grant date fair value (in dollars per share) | $ / shares			$ 65.52				$ 65.52		$ 31.55
Restricted Stock Units | Founder and CEO Long Term Performance Award | Founder CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation, equity instruments other than options | $	$ 232,200	$ 232,200	$ 202,500				$ 202,500
Allocated share based compensation | $			$ 12,300				$ 29,700
Share based compensation by share based payment arrangement maximum number of shares per employee (in shares)		11,500,000
Share-based compensation arrangement by share-based payment award, beginning Of award performance period, period after effective date		2 years
Share-based compensation arrangement by share-based payment award, number of tranches | tranche			7				7
Number of consecutive trading days for the stock hurdle price to be achieved		90 days
Share price (in dollars per share) | $ / shares	$ 165.00	$ 165.00
Grant date fair value (in dollars per share) | $ / shares	$ 20.19	$ 20.19
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment arrangement, unvested award, period for recognition							2 years 7 months 6 days
Unrecognized compensation, equity instruments other than options | $			$ 14,900				$ 14,900
Grant date fair value (in dollars per share) | $ / shares			$ 56.10				$ 56.10		$ 37.75
Minimum | Options | Two Thousand and Four Incentive Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options to be granted price as a percentage of fair value											85.00%
Minimum | Restricted Stock Units | Founder and CEO Long Term Performance Award | Founder CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment arrangement, unvested award, period for recognition	3 years 5 months 12 days
Maximum | Two Thousand and Twenty Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement contractual term of stock options									10 years
Maximum | Options | Two Thousand and Four Incentive Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement contractual term of stock options											10 years
Maximum | Options | Two Thousand and Seventeen Amended and Restated Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement contractual term of stock options										10 years
Maximum | Restricted Stock Units | Founder and CEO Long Term Performance Award | Founder CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment arrangement, unvested award, period for recognition	5 years 4 months 17 days
Holders of Ten Percent or More of The Voting Equity Capital | Two Thousand and Twenty Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement contractual term of stock options									5 years
Holders of Ten Percent or More of The Voting Equity Capital | Options | Two Thousand and Four Incentive Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of voting stock eligible for options											10.00%
Share based compensation by share based payment arrangement contractual term of stock options											5 years
Holders of Ten Percent or More of The Voting Equity Capital | Options | Two Thousand and Seventeen Amended and Restated Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement contractual term of stock options										5 years
Holders of Ten Percent or More of The Voting Equity Capital | Minimum | Two Thousand and Twenty Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options to be granted price as a percentage of fair value									100.00%
Holders of Ten Percent or More of The Voting Equity Capital | Minimum | Options | Two Thousand and Four Incentive Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options to be granted price as a percentage of fair value											110.00%
Holders of Ten Percent or More of The Voting Equity Capital | Minimum | Options | Two Thousand and Seventeen Amended and Restated Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options to be granted price as a percentage of fair value										110.00%