Unpaid claims - Additional information (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Liability for Unpaid Claims and Claims Adjustment Expense, Activity in Liability [Abstract]
Unpaid claims	$ 140,210	$ 93,615	$ 103,976	$ 77,886	$ 41,660
Prior years	$ 109,362	$ 55,559	$ 63,039	$ 37,742
Ratio of current year medical claims paid as a percent of current year net medical claims	0.882	0.796	0.830	0.831

Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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-Subparagraph (SX 210.7-03(13)(a))
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
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-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

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