Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 40,389	$ 36,355	$ 33,117
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	17,107	15,461	13,964
North America Commercial P&C Insurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,383	2,942	2,423
North America Commercial P&C Insurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	13,056	11,905	10,812
North America Commercial P&C Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	668	614	729
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	5,180	4,915	4,866
North America Personal P&C Insurance [Member] | Personal automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	811	781	822
North America Personal P&C Insurance [Member] | Personal homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,557	3,384	3,327
North America Personal P&C Insurance [Member] | Personal other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	812	750	717
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,838	2,338	1,822
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	10,803	10,441	9,285
Overseas General Insurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,382	3,105	2,468
Overseas General Insurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,232	3,114	2,738
Overseas General Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,169	2,113	2,098
Overseas General Insurance [Member] | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,020	2,109	1,981
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	922	798	698
Global Reinsurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	211	151	104
Global Reinsurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	503	457	421
Global Reinsurance [Member] | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned	208	190	173
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,539	2,402	2,482
Life Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,055	1,082	1,165
Life Insurance [Member] | Life [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,484	$ 1,320	$ 1,317