Information Provided In Connection With Outstanding Debt Of Subsidiaries (Condensed Consolidating Statement Of Operations) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Information Provided In Connection With Outstanding Debt Of Subsidiaries [Line Items]
Net premiums written	$ 4,716		$ 4,343		$ 12,418		$ 11,742
Net premiums earned	4,665		4,490		11,829		11,556
Net investment income	533		564		1,614		1,677
Net realized gains (losses) including OTTI	(60)		(760)		(194)		(878)
Losses and loss expenses	3,047		2,745		6,970		7,234
Policy benefits	130		83		379		282
Policy acquisition costs and administrative expenses	1,128		1,189		3,353		3,377
Interest (income) expense	63		62		187		187
Other (income) expense	(17)		89		14		88
Income tax expense	147		165		405		382
Net income (loss)	640		(39)		1,941		805
Comprehensive income	1,316		(189)		3,044		1,050
Parent Company [Member]
Information Provided In Connection With Outstanding Debt Of Subsidiaries [Line Items]
Net investment income	0				1		1
Equity in earnings of subsidiaries	616		(63)		1,845		742
Net realized gains (losses) including OTTI	(4)		2		18		0
Policy acquisition costs and administrative expenses	16		14		42		50
Interest (income) expense	(8)		(9)		(25)		(27)
Other (income) expense	(39)		(30)		(102)		(92)
Income tax expense	3		3		8		7
Net income (loss)	640		(39)		1,941		805
Comprehensive income	1,316		(189)		3,044		1,050
Guarantor Subsidiaries [Member]
Information Provided In Connection With Outstanding Debt Of Subsidiaries [Line Items]
Net premiums written	2,868		2,581		7,293		6,910
Net premiums earned	2,819		2,700		6,967		6,863
Net investment income	255		281		777		825
Net realized gains (losses) including OTTI	11		28		71		32
Losses and loss expenses	1,977		1,782		4,484		4,521
Policy benefits	77		22		217		121
Policy acquisition costs and administrative expenses	587		631		1,883		1,839
Interest (income) expense	67		66		191		199
Other (income) expense	17		85		51		121
Income tax expense	123		146		327		323
Net income (loss)	237		277		662		596
Comprehensive income	593		355		1,203		947
Other ACE Limited Subsidiaries And Eliminations [Member]
Information Provided In Connection With Outstanding Debt Of Subsidiaries [Line Items]
Net premiums written	1,848	[1]	1,762	[1]	5,125	[1]	4,832	[1]
Net premiums earned	1,846	[1]	1,790	[1]	4,862	[1]	4,693	[1]
Net investment income	278	[1]	283	[1]	836	[1]	851	[1]
Net realized gains (losses) including OTTI	(67)	[1]	(790)	[1]	(283)	[1]	(910)	[1]
Losses and loss expenses	1,070	[1]	963	[1]	2,486	[1]	2,713	[1]
Policy benefits	53	[1]	61	[1]	162	[1]	161	[1]
Policy acquisition costs and administrative expenses	525	[1]	544	[1]	1,428	[1]	1,488	[1]
Interest (income) expense	4	[1]	5	[1]	21	[1]	15	[1]
Other (income) expense	5	[1]	34	[1]	65	[1]	59	[1]
Income tax expense	21	[1]	16	[1]	70	[1]	52	[1]
Net income (loss)	379	[1]	(340)	[1]	1,183	[1]	146	[1]
Comprehensive income	23	[1]	(418)	[1]	642	[1]	(205)	[1]
Consolidation, Eliminations [Member]
Information Provided In Connection With Outstanding Debt Of Subsidiaries [Line Items]
Equity in earnings of subsidiaries	(616)	[2]	63	[2]	(1,845)	[2]	(742)	[2]
Net income (loss)	(616)	[2]	63	[2]	(1,845)	[2]	(742)	[2]
Comprehensive income	$ (616)	[2]	$ 63	[2]	$ (1,845)	[2]	$ (742)	[2]

[1] Includes all other subsidiaries of ACE Limited and intercompany eliminations.

[2] Includes ACE Limited parent company eliminations.