Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net premiums earned	$ 7,935	$ 8,327	$ 7,891	$ 7,137	$ 7,465	$ 7,908	$ 7,664	$ 7,027	$ 31,290	$ 30,064	$ 29,034
North America Commercial P&C Insurance
Segment Reporting Information [Line Items]
Net premiums earned									12,922	12,402	12,191
North America Commercial P&C Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,987	1,861	1,899
North America Commercial P&C Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									10,136	9,773	9,554
North America Commercial P&C Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									799	768	738
North American Personal P&C [Member]
Segment Reporting Information [Line Items]
Net premiums earned									4,694	4,593	4,399
North American Personal P&C [Member] | Automobiles [Member]
Segment Reporting Information [Line Items]
Net premiums earned									829	803	742
North American Personal P&C [Member] | Personal homeowner [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,183	3,127	3,014
North American Personal P&C [Member] | Insurance, Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									682	663	643
North America Agricultural Insurance
Segment Reporting Information [Line Items]
Net premiums earned									1,795	1,569	1,508
Overseas General Insurance
Segment Reporting Information [Line Items]
Net premiums earned									8,882	8,612	8,131
Overseas General Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,244	2,134	2,076
Overseas General Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,494	2,429	2,266
Overseas General Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,248	2,265	2,180
Overseas General Insurance | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,896	1,784	1,609
Segment Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned									654	670	704
Segment Global Reinsurance [Member] | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									131	123	132
Segment Global Reinsurance [Member] | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									381	377	374
Segment Global Reinsurance [Member] | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned									142	170	198
Life Insurance
Segment Reporting Information [Line Items]
Net premiums earned									2,343	2,218	2,101
Life Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,242	1,196	1,121
Life Insurance | Life Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									$ 1,101	$ 1,022	$ 980