Insurance and Contractholder Liabilities - U.S. Medical - Activity (Details) - U.S. Medical - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning balance	$ 2,892
Beginning balance		$ 2,697
Less: Reinsurance and other amounts recoverable	303
Less: Reinsurance and other amounts recoverable		264
Beginning balance, net	2,589
Beginning balance, net		2,433
Incurred costs related to:
Current year	18,935	18,169
Prior years	(126)	(159)
Total incurred	18,809	18,010
Paid costs related to:
Current year	16,061	15,555
Prior years	2,363	2,160
Total paid	18,424	17,715
Ending balance, net	2,974
Ending balance, net		2,728
Add: Reinsurance and other amounts recoverable	226
Add: Reinsurance and other amounts recoverable		333
Ending balance	$ 3,200
Ending balance		$ 3,061