Insurance and Contractholder Liabilities - Integrated Medical - Prior Year Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Favorable (unfavorable) impact of prior year development on pre-tax income	$ 97	$ 148
Integrated Medical [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Variance (unfavorable) in incurred costs related to prior years' claims payable	$ 173	$ 227	$ 70
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.90%	1.30%
Favorable (unfavorable) impact of prior year development on shareholders' net income	$ 77	$ 96
Completion Factors [Member] | Integrated Medical [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Variance (unfavorable) in incurred costs related to prior years' claims payable	$ 92	$ 87
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.50%	0.60%
Medical Cost Trend [Member] | Integrated Medical [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Variance (unfavorable) in incurred costs related to prior years' claims payable	$ 72	$ 131
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.40%	0.70%
Other [Member] | Integrated Medical [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Abstract]
Variance (unfavorable) in incurred costs related to prior years' claims payable	$ 9	$ 9
Variance (unfavorable) in incurred costs related to prior years' claims payable, percentage	0.00%	0.00%