Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 36,355	$ 33,117	$ 31,290
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	15,461	13,964	12,922
North America Commercial P&C Insurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,942	2,423	1,987
North America Commercial P&C Insurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	11,905	10,812	10,136
North America Commercial P&C Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	614	729	799
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	4,915	4,866	4,694
North America Personal P&C Insurance [Member] | Personal automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	781	822	829
North America Personal P&C Insurance [Member] | Personal homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,384	3,327	3,183
North America Personal P&C Insurance [Member] | Personal other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	750	717	682
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,338	1,822	1,795
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	10,441	9,285	8,882
Overseas General Insurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,105	2,468	2,244
Overseas General Insurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,114	2,738	2,494
Overseas General Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,113	2,098	2,248
Overseas General Insurance [Member] | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,109	1,981	1,896
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	798	698	654
Global Reinsurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	151	104	131
Global Reinsurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	457	421	381
Global Reinsurance [Member] | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned	190	173	142
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,402	2,482	2,343
Life Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,082	1,165	1,242
Life Insurance [Member] | Life [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,320	$ 1,317	$ 1,101