Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,478	$ 4,204
Insurance receivables	2,640	2,481
Deferred acquisition costs	708	662
Goodwill	148	147
Deferred non-insurance warranty acquisition expense	3,068	2,840
Insurance reserves
Claim and claim adjustment expenses	22,706	21,720
Unearned premiums	5,119	4,583
Future policy benefits	13,318	12,311
Deferred non-insurance warranty revenue	4,023	3,779
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	886	575
Insurance receivables	1,052	971
Deferred acquisition costs	330	311
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	3,068	2,840
Insurance reserves
Claim and claim adjustment expenses	5,748	5,238
Unearned premiums	2,635	2,337
Future policy benefits	0	0
Deferred non-insurance warranty revenue	4,023	3,779
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	922	855
Insurance receivables	1,254	1,210
Deferred acquisition costs	281	257
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	9,041	8,656
Unearned premiums	1,824	1,626
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	302	247
Insurance receivables	328	284
Deferred acquisition costs	97	94
Goodwill	31	30
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	2,091	1,876
Unearned premiums	546	495
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	390	385
Insurance receivables	4	16
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,743	3,716
Unearned premiums	114	125
Future policy benefits	13,318	12,311
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	1,978	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	2,083	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