Insurance and Contractholder Liabilities - Summary (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Insurance and Contractholder Liabilities [Line Items]
Contractholder deposit funds, current	$ 626	$ 641
Future policy benefits, current	588	740
Unearned premiums, current	402	348
Total insurance and contractholder liabilities, current	6,930	6,801
Contractholder deposit funds, non-current	7,257	7,365
Future policy benefits, non-current	9,127	8,981
Unearned premiums, non-current	374	379
Total insurance and contractholder liabilities, non-current	20,118	19,974
Contractholder deposit funds	7,883	8,006	$ 8,121
Future policy benefits	9,715	9,721	9,707
Unearned premiums	776	727	1,244
Total insurance and contractholder liabilities	27,048	26,775	27,323
Integrated Medical [Member]
Insurance and Contractholder Liabilities [Line Items]
Unpaid claims and claim expenses, current	2,862	2,678
Unpaid claims and claim expenses, non-current	19	19
Total liability for unpaid claims and claims expenses	2,881	2,697	2,749	$ 2,420
Other Segments [Member]
Insurance and Contractholder Liabilities [Line Items]
Unpaid claims and claim expenses, current	2,452	2,394
Unpaid claims and claim expenses, non-current	3,341	3,230
Total liability for unpaid claims and claims expenses	$ 5,793	$ 5,624	$ 5,502