Unpaid Claims - Additional information (Details) $ in Thousands	6 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Liability for Unpaid Claims and Claims Adjustment Expense, Activity in Liability [Abstract]
Unpaid claims	$ 140,210	$ 140,210	$ 93,615	$ 103,976	$ 77,886
Incurred claims paid		1,073,141	394,811
Favorable development		$ 17,095	$ (15,401)
Ratio of current year medical claims paid as a percent of current year net medical claims		0.882	0.796
DC Model, percentage of incurred claims	42.40%