Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 5,133	$ 4,478
Insurance receivables	2,685	2,640
Deferred acquisition costs	741	708
Goodwill	148	148
Deferred non-insurance warranty acquisition expense	3,149	3,068
Insurance reserves
Claim and claim adjustment expenses	23,056	22,706
Unearned premiums	5,319	5,119
Future policy benefits	12,772	13,318
Deferred non-insurance warranty revenue	4,119	4,023
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	957	886
Insurance receivables	1,004	1,052
Deferred acquisition costs	340	330
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	3,149	3,068
Insurance reserves
Claim and claim adjustment expenses	5,916	5,748
Unearned premiums	2,666	2,635
Future policy benefits	0	0
Deferred non-insurance warranty revenue	4,119	4,023
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	859	848
Insurance receivables	1,322	1,254
Deferred acquisition costs	299	281
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	8,449	8,250
Unearned premiums	1,930	1,824
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	300	302
Insurance receivables	357	328
Deferred acquisition costs	102	97
Goodwill	31	31
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,142	2,091
Unearned premiums	588	546
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	389	390
Insurance receivables	2	4
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,726	3,743
Unearned premiums	135	114
Future policy benefits	12,772	13,318
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,628	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	2,823	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