Insurance and Contractholder Liabilities - Integrated Medical - Summary (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance and Contractholder Liabilities [Line Items]
Contractholder deposit funds, current	$ 641	$ 713
Future policy benefits, current	740	706
Unearned premiums, current	348	319
Total insurance and contractholder liabilities, current	6,801	6,317
Contractholder deposit funds, non-current	7,365	7,483
Future policy benefits, non-current	8,981	9,334
Unearned premiums, non-current	379	405
Total insurance and contractholder liabilities, non-current	19,974	20,530
Contractholder deposit funds	8,006	8,196
Future policy benefits	9,721	10,040
Unearned premiums	727	724
Total insurance and contractholder liabilities	$ 26,775	26,847
Maximum [Member]
Insurance and Contractholder Liabilities [Line Items]
Interest rate assumptions for Future policy benefits obligations	9.00%
Minimum [Member]
Insurance and Contractholder Liabilities [Line Items]
Interest rate assumptions for Future policy benefits obligations	1.00%
Integrated Medical [Member]
Insurance and Contractholder Liabilities [Line Items]
Unpaid claims and claim expenses, current	$ 2,678	2,401
Unpaid claims and claim expenses, non-current	19	19
Total liability for unpaid claims and claims expenses	2,697	2,420	$ 2,261	$ 2,105
Other Segments [Member]
Insurance and Contractholder Liabilities [Line Items]
Unpaid claims and claim expenses, current	2,394	2,178
Unpaid claims and claim expenses, non-current	3,230	3,289
Total liability for unpaid claims and claims expenses	$ 5,624	$ 5,467