Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,394	$ 4,204
Insurance receivables	2,559	2,481
Deferred acquisition costs	697	662
Goodwill	146	147
Deferred non-insurance warranty acquisition expense	2,998	2,840
Insurance reserves
Claim and claim adjustment expenses	22,534	21,720
Unearned premiums	5,020	4,583
Future policy benefits	12,978	12,311
Deferred non-insurance warranty revenue	3,951	3,779
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	855	575
Insurance receivables	1,015	971
Deferred acquisition costs	324	311
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	2,998	2,840
Insurance reserves
Claim and claim adjustment expenses	5,698	5,238
Unearned premiums	2,550	2,337
Future policy benefits	0	0
Deferred non-insurance warranty revenue	3,951	3,779
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	948	855
Insurance receivables	1,267	1,210
Deferred acquisition costs	285	257
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	9,054	8,656
Unearned premiums	1,847	1,626
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	253	247
Insurance receivables	269	284
Deferred acquisition costs	88	94
Goodwill	29	30
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	1,975	1,876
Unearned premiums	502	495
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	401	385
Insurance receivables	7	16
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,766	3,716
Unearned premiums	121	125
Future policy benefits	12,978	12,311
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	1,937	2,142
Insurance receivables	1	0
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,041	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	0	0
Future policy benefits	0	0
Deferred non-insurance warranty revenue	$ 0	$ 0