Business Segments (Balance sheet information) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 5,349	$ 4,478
Insurance receivables	2,829	2,640
Deferred acquisition costs	721	708
Goodwill	148	148
Deferred non-insurance warranty acquisition expense	3,418	3,068
Insurance reserves
Claim and claim adjustment expenses	23,832	22,706
Unearned premiums	5,577	5,119
Future policy benefits	13,198	13,318
Deferred non-insurance warranty revenue	4,443	4,023
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	1,153	886
Insurance receivables	1,087	1,052
Deferred acquisition costs	354	330
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	3,418	3,068
Insurance reserves
Claim and claim adjustment expenses	6,293	5,748
Unearned premiums	2,882	2,635
Future policy benefits	0	0
Deferred non-insurance warranty revenue	4,443	4,023
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	936	848
Insurance receivables	1,422	1,254
Deferred acquisition costs	275	281
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	8,856	8,250
Unearned premiums	2,020	1,824
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	347	302
Insurance receivables	313	328
Deferred acquisition costs	92	97
Goodwill	31	31
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,251	2,091
Unearned premiums	559	546
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	410	390
Insurance receivables	5	4
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,703	3,743
Unearned premiums	116	114
Future policy benefits	13,198	13,318
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,503	2,052
Insurance receivables	2	2
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,729	2,874
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