Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) segment	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Summary of Significant Accounting Policies
Number of reportable segments \| segment	1	1
Premium receivable, allowance for doubtful accounts	$ 1.0	$ 0.5	$ 0.2
Amortization of net deferred acquisition costs		4.9	2.1	$ 0.7
Matching contributions		0.5	0.3	0.0
Advertising expenses		$ 58.3	$ 76.0	$ 36.2
Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Requisite service period		4 years

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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 amortization expense (reversal of expense) for deferred policy acquisition costs.

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Amount of discretionary contributions made by an employer to a defined contribution plan.

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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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The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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