Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,455		$ 4,290
Insurance receivables	2,365		2,336
Deferred acquisition costs	633		634
Goodwill	146		148
Deferred non-insurance warranty acquisition expense	2,513	$ 2,094	212
Insurance reserves
Claim and claim adjustment expenses	21,984		22,004
Unearned premiums	4,183		4,029
Future policy benefits	10,597		11,179
Deferred non-insurance warranty revenue	3,402	$ 2,941	972
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	649		671
Insurance receivables	947		969
Deferred acquisition costs	308		318
Goodwill	117		117
Deferred non-insurance warranty acquisition expense	2,513		212
Insurance reserves
Claim and claim adjustment expenses	5,465		5,669
Unearned premiums	2,132		2,020
Future policy benefits	0		0
Deferred non-insurance warranty revenue	3,402		972
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	795		654
Insurance receivables	1,277		1,103
Deferred acquisition costs	230		223
Goodwill	0		0
Deferred non-insurance warranty acquisition expense	0		0
Insurance reserves
Claim and claim adjustment expenses	8,743		8,764
Unearned premiums	1,454		1,409
Future policy benefits	0		0
Deferred non-insurance warranty revenue	0		0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	250		212
Insurance receivables	284		254
Deferred acquisition costs	95		93
Goodwill	29		31
Deferred non-insurance warranty acquisition expense	0		0
Insurance reserves
Claim and claim adjustment expenses	1,750		1,636
Unearned premiums	475		472
Future policy benefits	0		0
Deferred non-insurance warranty revenue	0		0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	414		438
Insurance receivables	9		8
Deferred acquisition costs	0		0
Goodwill	0		0
Deferred non-insurance warranty acquisition expense	0		0
Insurance reserves
Claim and claim adjustment expenses	3,601		3,499
Unearned premiums	122		128
Future policy benefits	10,597		11,179
Deferred non-insurance warranty revenue	0		0
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,347		2,315
Insurance receivables	(152)		2
Deferred acquisition costs	0		0
Goodwill	0		0
Deferred non-insurance warranty acquisition expense	0		0
Insurance reserves
Claim and claim adjustment expenses	2,425		2,436
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