Insurance and Contractholder Liabilities - Variances in Incurred Costs Related to Prior Years' Unpaid Claims and Claims Expenses (Details) - Integrated Medical - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) variance, amount	$ 152	$ 117
Favorable (unfavorable) variance, percentage	0.60%	0.50%
Actual completion factors
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) variance, amount	$ 63	$ 55
Favorable (unfavorable) variance, percentage	0.30%	0.20%
Medical cost trend
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) variance, amount	$ 89	$ 62
Favorable (unfavorable) variance, percentage	0.30%	0.30%