Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net premiums earned	$ 33,117	$ 31,290	$ 30,064
North America Commercial P&C Insurance
Segment Reporting Information [Line Items]
Net premiums earned	13,964	12,922	12,402
North America Commercial P&C Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,423	1,987	1,861
North America Commercial P&C Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	10,812	10,136	9,773
North America Commercial P&C Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	729	799	768
North American Personal P&C [Member]
Segment Reporting Information [Line Items]
Net premiums earned	4,866	4,694	4,593
North American Personal P&C [Member] | Automobiles [Member]
Segment Reporting Information [Line Items]
Net premiums earned	822	829	803
North American Personal P&C [Member] | Personal homeowner [Member]
Segment Reporting Information [Line Items]
Net premiums earned	3,327	3,183	3,127
North American Personal P&C [Member] | Insurance, Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	717	682	663
North America Agricultural Insurance
Segment Reporting Information [Line Items]
Net premiums earned	1,822	1,795	1,569
Overseas General Insurance
Segment Reporting Information [Line Items]
Net premiums earned	9,285	8,882	8,612
Overseas General Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,468	2,244	2,134
Overseas General Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,738	2,494	2,429
Overseas General Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	2,098	2,248	2,265
Overseas General Insurance | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,981	1,896	1,784
Segment Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	698	654	670
Segment Global Reinsurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned	104	131	123
Segment Global Reinsurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	421	381	377
Segment Global Reinsurance [Member] | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned	173	142	170
Life Insurance
Segment Reporting Information [Line Items]
Net premiums earned	2,482	2,343	2,218
Life Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,165	1,242	1,196
Life Insurance | Life Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,317	$ 1,101	$ 1,022