Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 45,712	$ 40,360	$ 36,292
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	18,416	17,107	15,461
North America Commercial P&C Insurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,985	3,383	2,942
North America Commercial P&C Insurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	13,764	13,056	11,905
North America Commercial P&C Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	667	668	614
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	5,536	5,180	4,915
North America Personal P&C Insurance [Member] | Personal automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	859	811	781
North America Personal P&C Insurance [Member] | Personal homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,833	3,557	3,384
North America Personal P&C Insurance [Member] | Insurance, Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	844	812	750
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,169	2,838	2,338
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	12,231	10,803	10,441
Overseas General Insurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,831	3,382	3,105
Overseas General Insurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,526	3,232	3,114
Overseas General Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,469	2,169	2,113
Overseas General Insurance [Member] | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,405	2,020	2,109
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	962	922	798
Global Reinsurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	331	211	151
Global Reinsurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	472	503	457
Global Reinsurance [Member] | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned	159	208	190
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	5,398	3,510	2,339
Life Insurance [Member] | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,097	2,055	1,082
Life Insurance [Member] | Life [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 2,301	$ 1,455	$ 1,257