Insurance and Contractholder Liabilities - Variances in Incurred Costs Related to Prior Years' Unpaid Claims and Claims Expenses (Details) - U.S. Medical - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) variance, amount	$ 115	$ 165	$ 173
Favorable (unfavorable) variance, percentage	0.50%	0.80%
Actual completion factors
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) variance, amount	$ 42	$ 90
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	$ 73	$ 75
Favorable (unfavorable) variance, percentage	0.30%	0.40%