Commitments and Contingencies - Concentration Risk (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Concentration Risk [Line Items]
Percentage of cash and cash equivalents held with FDIC participating bank		99.50%		99.50%
Cash and cash equivalents (including restricted cash)	[1]	$ 281,755	$ 400,551	$ 281,755	$ 400,551	$ 361,581	$ 128,531
Percentage of cash held in international banks (less than)		0.50%		0.50%
Discontinued Operations, Disposed of by Sale | True Health
Concentration Risk [Line Items]
Revenue		$ 0	29,558	$ 44,833	87,486
Premiums | Discontinued Operations, Disposed of by Sale | True Health
Concentration Risk [Line Items]
Revenue		$ 0	$ 29,487	$ 44,795	$ 87,136
Cook County Health and Hospitals System | Customer Concentration Risk | Accounts Receivable | Customer Receivable
Concentration Risk [Line Items]
Concentration risk				43.70%		61.50%
Cook County Health and Hospitals System | Customer Concentration Risk | Revenues
Concentration Risk [Line Items]
Concentration risk		29.00%	23.10%	29.00%	22.00%
Florida Blue Medicare, Inc. | Customer Concentration Risk | Accounts Receivable | Customer Receivable
Concentration Risk [Line Items]
Concentration risk				33.30%
Florida Blue Medicare, Inc. | Customer Concentration Risk | Revenues
Concentration Risk [Line Items]
Concentration risk		13.80%		14.20%
Passport | Customer Concentration Risk | Revenues
Concentration Risk [Line Items]
Concentration risk			22.90%		25.00%

[1] As a result of the closing of the sale of True Health SPA, the consolidated statements of operations, consolidated balance sheets, and related financial information reflect the Company’s operations and assets and liabilities of True Health as discontinued operations for all periods presented. Cash flows and comprehensive income have not been adjusted and are included in the consolidated statements of cash flows and consolidated statements of comprehensive income (loss) for all periods presented. See Note 5.