Insurance and Contractholder Liabilities - Variances in Incurred Costs Related to Prior Years' Unpaid Claims and Claims Expenses (Details) - U.S. Medical - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) variance, amount	$ 126	$ 159
Favorable (unfavorable) variance, percentage	0.50%	0.70%
Actual completion factors
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) variance, amount	$ 47	$ 87
Favorable (unfavorable) variance, percentage	0.20%	0.40%
Medical cost trend
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) variance, amount	$ 79	$ 72
Favorable (unfavorable) variance, percentage	0.30%	0.30%