Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,351	$ 4,204
Insurance receivables	2,532	2,481
Deferred acquisition costs	683	662
Goodwill	145	147
Deferred non-insurance warranty acquisition expense	2,905	2,840
Insurance reserves
Claim and claim adjustment expenses	21,872	21,720
Unearned premiums	4,745	4,583
Future policy benefits	11,734	12,311
Deferred non-insurance warranty revenue	3,848	3,779
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	793	575
Insurance receivables	953	971
Deferred acquisition costs	316	311
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	2,905	2,840
Insurance reserves
Claim and claim adjustment expenses	5,472	5,238
Unearned premiums	2,363	2,337
Future policy benefits	0	0
Deferred non-insurance warranty revenue	3,848	3,779
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	883	855
Insurance receivables	1,267	1,210
Deferred acquisition costs	277	257
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	8,704	8,656
Unearned premiums	1,751	1,626
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	233	247
Insurance receivables	302	284
Deferred acquisition costs	90	94
Goodwill	28	30
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	1,818	1,876
Unearned premiums	495	495
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	370	385
Insurance receivables	8	16
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,712	3,716
Unearned premiums	137	125
Future policy benefits	11,734	12,311
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,072	2,142
Insurance receivables	2	0
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,166	2,234
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