Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,263	$ 4,455
Insurance receivables	2,690	2,365
Deferred acquisition costs	681	633
Goodwill	146	146
Deferred non-insurance warranty acquisition expense	2,678	2,513
Insurance reserves
Claim and claim adjustment expenses	21,729	21,984
Unearned premiums	4,648	4,183
Future policy benefits	11,537	10,597
Deferred non-insurance warranty revenue	3,595	3,402
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	696	649
Insurance receivables	1,002	947
Deferred acquisition costs	316	308
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	2,678	2,513
Insurance reserves
Claim and claim adjustment expenses	5,396	5,465
Unearned premiums	2,261	2,132
Future policy benefits	0	0
Deferred non-insurance warranty revenue	3,595	3,402
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	766	795
Insurance receivables	1,362	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	8,580	8,743
Unearned premiums	1,721	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	318	284
Deferred acquisition costs	101	95
Goodwill	29	29
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	1,800	1,750
Unearned premiums	534	475
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	399	414
Insurance receivables	7	9
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,710	3,601
Unearned premiums	133	122
Future policy benefits	11,537	10,597
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,160	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	2,243	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