Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 5,220	$ 4,478
Insurance receivables	2,988	2,640
Deferred acquisition costs	721	708
Goodwill	148	148
Deferred non-insurance warranty acquisition expense	3,305	3,068
Insurance reserves
Claim and claim adjustment expenses	23,480	22,706
Unearned premiums	5,592	5,119
Future policy benefits	13,285	13,318
Deferred non-insurance warranty revenue	4,309	4,023
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	1,084	886
Insurance receivables	1,108	1,052
Deferred acquisition costs	340	330
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	3,305	3,068
Insurance reserves
Claim and claim adjustment expenses	6,178	5,748
Unearned premiums	2,767	2,635
Future policy benefits	0	0
Deferred non-insurance warranty revenue	4,309	4,023
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	814	848
Insurance receivables	1,509	1,254
Deferred acquisition costs	277	281
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	8,533	8,250
Unearned premiums	2,086	1,824
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	339	302
Insurance receivables	365	328
Deferred acquisition costs	104	97
Goodwill	31	31
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,225	2,091
Unearned premiums	616	546
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	414	390
Insurance receivables	6	4
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,754	3,743
Unearned premiums	123	114
Future policy benefits	13,285	13,318
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,569	2,052
Insurance receivables	0	2
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,790	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