Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,306	$ 4,455
Insurance receivables	2,476	2,365
Deferred acquisition costs	664	633
Goodwill	147	146
Deferred non-insurance warranty acquisition expense	2,576	2,513
Insurance reserves
Claim and claim adjustment expenses	21,836	21,984
Unearned premiums	4,422	4,183
Future policy benefits	11,078	10,597
Deferred non-insurance warranty revenue	3,472	3,402
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	674	649
Insurance receivables	933	947
Deferred acquisition costs	315	308
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	2,576	2,513
Insurance reserves
Claim and claim adjustment expenses	5,470	5,465
Unearned premiums	2,201	2,132
Future policy benefits	0	0
Deferred non-insurance warranty revenue	3,472	3,402
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	761	795
Insurance receivables	1,234	1,277
Deferred acquisition costs	247	230
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	8,623	8,743
Unearned premiums	1,553	1,454
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	237	250
Insurance receivables	302	284
Deferred acquisition costs	102	95
Goodwill	30	29
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	1,787	1,750
Unearned premiums	528	475
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	408	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	3,645	3,601
Unearned premiums	142	122
Future policy benefits	11,078	10,597
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,226	2,347
Insurance receivables	0	(152)
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,311	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	(2)	0
Future policy benefits	0	0
Deferred non-insurance warranty revenue	$ 0	$ 0