Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Sep. 30, 2021 USD ($)
Accounting Policies [Line Items]
Number of operating segments \| segment	1
Number of reportable segments \| segment	1
Deferred rent	$ 28,760	$ 25,172	$ 44,110
Prepaid rent	9,907	13,882	$ 3,009
Gain (loss) from foreign currency transactions	600	(5,700)
Advertising expenses	$ 4,900	$ 1,200
Internal-use software
Accounting Policies [Line Items]
Useful life	2 years
Stock Options
Accounting Policies [Line Items]
Requisite service period	4 years
Contractual term	10 years

Accounting Policies [Line Items]

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Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line.

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Amount of excess of rental payment required by lease over rental income recognized, classified as noncurrent.

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Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement.

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- Definition

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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- Definition

Amount of asset related to consideration paid in advance for rent that provides economic benefits within a future period of one year or the normal operating cycle, if longer.

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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