Health Care Costs Payable - Reconciliation of Health Care Net Incurred and Paid Claims Development to Health Care Costs Payable Liability (Details) - Health Insurance Product Line - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration health care costs payable, net of reinsurance	$ 7,768
Reinsurance recoverables	10	$ 5	$ 4	$ 0
Premium deficiency reserve	11	4	16
Insurance lines other than short duration	147
Total health care costs payable	$ 7,936	$ 6,879	$ 6,147	$ 5