Summary of significant accounting policies (Narrative) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of significant accounting policies [Line Items]
Deferred Marketing Costs, Amortization Period	10 years
Deferred marketing costs reported in Deferred policy acquisition costs	$ 274	$ 236
Amortization expense for deferred marketing costs	156	128	115
Recoverable from unrated reinsurers, ceded reserve, default factor (percent)	34.00%
Reinsurance business assumed	32	35
Percentage of fair value of loaned securities	102.00%
Maximum overdraft balance guaranteed by ACE Ltd	300
Quality assessment threshold used in goodwill impairment testing	50.00%
Loss and loss expenses reserves, net of discount	58
Gross liability for the amounts due to claimants	640
Reinsurance recoverables for amounts due from life insurance companies	582
Deposit assets reflected in Other assets	138	133
Reinsurance deposit liabilities included in Deposit liabilities	283	318
Contract holder deposit funds included in Deposit liabilities	548	345
Net operating results of ESIS included within Administrative expenses	23	21	85
Cumulative effect of change in accounting principle		45	23
New accounting pronouncement effect of change on earnings per share		$ 0.13	$ 0.07
Deferred Policy Acquisition Costs
Summary of significant accounting policies [Line Items]
Cumulative effect of change in accounting principle		213
Retained Earnings
Summary of significant accounting policies [Line Items]
Cumulative effect of change in accounting principle		$ 181
Minimum
Summary of significant accounting policies [Line Items]
Amortization period for value of reinsurance business assumed	7 years
Finite-lived intangible asset useful life	4 years
Interest rates used in calculating reserves	1.00%	1.00%
Reinsurance Premiums, Amortization Period	1 year
Maximum
Summary of significant accounting policies [Line Items]
Amortization period for value of reinsurance business assumed	40 years
Finite-lived intangible asset useful life	20 years
Interest rates used in calculating reserves	4.50%	6.00%
Reinsurance Premiums, Amortization Period	3 years