Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Net premiums earned	$ 7,465	$ 7,908	$ 7,664	$ 7,027	$ 7,218	$ 7,807	$ 7,237	$ 6,772	$ 30,064	$ 29,034		$ 28,749
North America Commercial P&C Insurance
Segment Reporting Information [Line Items]
Net premiums earned									12,402	12,191		12,217
North America Commercial P&C Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,861	1,899		1,963
North America Commercial P&C Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									9,773	9,554		9,552
North America Commercial P&C Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									768	738		702
North American Personal P&C [Member]
Segment Reporting Information [Line Items]
Net premiums earned									4,593	4,399		4,319
North American Personal P&C [Member] | Automobiles [Member]
Segment Reporting Information [Line Items]
Net premiums earned									803	742		699
North American Personal P&C [Member] | Personal homeowner [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,127	3,014		3,007
North American Personal P&C [Member] | Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									663	643		613
North America Agricultural Insurance
Segment Reporting Information [Line Items]
Net premiums earned									1,569	1,508		1,316
Overseas General Insurance
Segment Reporting Information [Line Items]
Net premiums earned									8,612	8,131		8,132
Overseas General Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,134	2,076	[1]	2,133	[1]
Overseas General Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,429	2,266	[1]	2,177	[1]
Overseas General Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,265	2,180		2,196
Overseas General Insurance | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,784	1,609		1,626
Global Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned									670	704		710
Global Reinsurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									123	132		118
Global Reinsurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									377	374		407
Global Reinsurance | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned									170	198		185
Life Insurance
Segment Reporting Information [Line Items]
Net premiums earned									2,218	2,101		2,055
Life Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,196	1,121		1,053
Life Insurance | Life Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									$ 1,022	$ 980		$ 1,002

[1] For the Year Ended December 31 (in millions of U.S. dollars)2018 2017 2016North America Commercial P&C Insurance Property & other short-tail lines$1,861 $1,899 $1,963Casualty & all other9,773 9,554 9,552A&H768 738 702Total North America Commercial P&C Insurance12,402 12,191 12,217North America Personal P&C Insurance Personal automobile803 742 699Personal homeowners3,127 3,014 3,007Personal other663 643 613Total North America Personal P&C Insurance4,593 4,399 4,319North America Agricultural Insurance1,569 1,508 1,316Overseas General Insurance Property & other short-tail lines2,134 2,076 2,133Casualty & all other2,429 2,266 2,177Personal lines1,784 1,609 1,626A&H2,265 2,180 2,196Total Overseas General Insurance8,612 8,131 8,132Global Reinsurance Property & other short-tail lines123 132 118Property catastrophe170 198 185Casualty & all other377 374 407Total Global Reinsurance670 704 710Life Insurance Life1,022 980 1,002A&H1,196 1,121 1,053Total Life Insurance2,218 2,101 2,055Total net premiums earned$30,064 $29,034 $28,749