Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 2,237	$ 1,818	$ 1,659
Future policy benefits and contractholder deposit funds	18,236	18,106	17,922
Medical claims payable and unpaid claims	7,887	7,449	6,929
Unearned premiums	724	634	629
Premiums	32,307	30,626	29,642
Net investment income	1,226	1,147	1,153
Benefit expenses	25,406	24,486	23,290
Amortization of deferred policy acquisition expenses	322	292	286
Other operating expenses	12,282	11,911	10,973
Reduction of operating expenses resulting from tax reform	56
Corporate [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	0	0	0
Future policy benefits and contractholder deposit funds	0	0	0
Medical claims payable and unpaid claims	0	0	0
Unearned premiums	0	0	0
Premiums	(12)	0	0
Net investment income	30	21	4
Benefit expenses	(12)	0	0
Amortization of deferred policy acquisition expenses	0	0	0
Other operating expenses	815	528	502
Global Health Care [Member] | Operating Segments [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	15	16	11
Future policy benefits and contractholder deposit funds	157	161	169
Medical claims payable and unpaid claims	2,719	2,532	2,355
Unearned premiums	213	170	145
Premiums	24,538	23,295	22,696
Net investment income	378	315	340
Benefit expenses	19,967	19,009	18,354
Amortization of deferred policy acquisition expenses	56	47	53
Other operating expenses	9,417	9,411	8,621
Global Supplemental Benefits [Member] | Operating Segments [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	2,176	1,752	1,593
Future policy benefits and contractholder deposit funds	3,746	3,225	3,006
Medical claims payable and unpaid claims	484	384	353
Unearned premiums	490	435	453
Premiums	3,684	3,226	3,000
Net investment income	122	110	103
Benefit expenses	2,033	1,784	1,659
Amortization of deferred policy acquisition expenses	259	238	227
Other operating expenses	1,115	1,030	963
Group Disability And Life [Member] | Operating Segments [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	1	1	1
Future policy benefits and contractholder deposit funds	1,686	1,786	1,714
Medical claims payable and unpaid claims	4,491	4,342	4,006
Unearned premiums	7	13	13
Premiums	3,985	4,002	3,843
Net investment income	350	343	337
Benefit expenses	3,076	3,354	2,934
Amortization of deferred policy acquisition expenses	1	1	1
Other operating expenses	967	918	861
Other Operations [Member] | Operating Segments [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	45	49	54
Future policy benefits and contractholder deposit funds	12,647	12,934	13,033
Medical claims payable and unpaid claims	193	191	215
Unearned premiums	14	16	18
Premiums	112	103	103
Net investment income	346	358	369
Benefit expenses	342	339	343
Amortization of deferred policy acquisition expenses	6	6	5
Other operating expenses	(32)	$ 24	$ 26
Reduction of operating expenses resulting from tax reform	$ 56