Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,459	$ 4,204
Insurance receivables	2,788	2,481
Deferred acquisition costs	699	662
Goodwill	145	147
Deferred non-insurance warranty acquisition expense	2,916	2,840
Insurance reserves
Claim and claim adjustment expenses	22,270	21,720
Unearned premiums	4,996	4,583
Future policy benefits	12,596	12,311
Deferred non-insurance warranty revenue	3,852	3,779
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	897	575
Insurance receivables	1,035	971
Deferred acquisition costs	315	311
Goodwill	117	117
Deferred non-insurance warranty acquisition expense	2,916	2,840
Insurance reserves
Claim and claim adjustment expenses	5,696	5,238
Unearned premiums	2,424	2,337
Future policy benefits	0	0
Deferred non-insurance warranty revenue	3,852	3,779
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	925	855
Insurance receivables	1,429	1,210
Deferred acquisition costs	293	257
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	8,830	8,656
Unearned premiums	1,927	1,626
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	246	247
Insurance receivables	313	284
Deferred acquisition costs	91	94
Goodwill	28	30
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	1,906	1,876
Unearned premiums	517	495
Future policy benefits	0	0
Deferred non-insurance warranty revenue	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	407	385
Insurance receivables	10	16
Deferred acquisition costs	0	0
Goodwill	0	0
Deferred non-insurance warranty acquisition expense	0	0
Insurance reserves
Claim and claim adjustment expenses	3,751	3,716
Unearned premiums	128	125
Future policy benefits	12,596	12,311
Deferred non-insurance warranty revenue	0	0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	1,984	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	2,087	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