Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,092	$ 4,455
Insurance receivables	2,504	2,365
Deferred acquisition costs	668	633
Goodwill	145	146
Deferred non-insurance warranty acquisition expense	2,772	2,513
Insurance reserves
Claim and claim adjustment expenses	21,596	21,984
Unearned premiums	4,608	4,183
Future policy benefits	12,305	10,597
Deferred non-insurance warranty revenue	3,707	3,402
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	599	649
Insurance receivables	992	947
Deferred acquisition costs	314	308
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	2,772	2,513
Insurance reserves
Claim and claim adjustment expenses	5,285	5,465
Unearned premiums	2,329	2,132
Future policy benefits	0	0
Deferred non-insurance warranty revenue	3,707	3,402
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	759	795
Insurance receivables	1,243	1,277
Deferred acquisition costs	264	230
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	8,610	8,743
Unearned premiums	1,679	1,454
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	242	250
Insurance receivables	260	284
Deferred acquisition costs	90	95
Goodwill	28	29
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	1,802	1,750
Unearned premiums	475	475
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	394	414
Insurance receivables	8	9
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,707	3,601
Unearned premiums	126	122
Future policy benefits	12,305	10,597
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,098	2,347
Insurance receivables	1	(152)
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,192	2,425
Unearned premiums	0	0
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Eliminations
Segment Reporting Information [Line Items]
Reinsurance receivables	0	0
Insurance receivables	0	0
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	0	0
Unearned premiums	(1)	0
Future policy benefits	0	0
Deferred non-insurance warranty revenue	$ 0	$ 0