Summary of significant accounting policies (Narrative) (Detail) - USD ($) $ in Millions		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2016		Dec. 31, 2015	Jan. 14, 2016	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Chubb integration expenses		$ 310	$ 492				$ 33
Affiliated notional cash pooling program		300	0	[1]	$ 0	[1]
Reinsurance business assumed		$ 18	20
Recoverable from unrated reinsurers, ceded reserve, default factor (percent)		34.00%
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense			455		455
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Deposit assets reflected in Other assets		$ 89	93		93
Unpaid losses and loss expenses		63,179	60,540		60,540		37,303		$ 38,315
Deposit liabilities included in Deposit liabilities		100	108		108
Borrowings under Guaranteed Investment Agreements		1,800	1,500		1,500
Summary of significant accounting policies [Line Items]
Deferred Policy Acquisition Costs, Amortization Expense		5,781	5,904				2,941
Deferred Advertising Costs		$ 271	256		256
Deferred Marketing Costs, Amortization Period		10 years
Percentage of fair value of loaned securities		102.00%
Quality assessment threshold used in goodwill impairment testing		50.00%
Supplemental Information for Property, Casualty Insurance Underwriters, Reserves for Unpaid Claims and Claims Adjustment Expense		$ 49,165	47,832		47,832		26,562
Entity Well-known Seasoned Issuer		Yes
Net operating results of ESIS included within Administrative expenses		$ 38	32				30
Property, Plant and Equipment, Net		1,300	1,200		1,200
Finite-Lived Intangible Assets, Net		3,500	3,805		3,805
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		48
Income tax benefit related to 2017 Tax Act		(450)	0				0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		$ (450)
Tax Year 2018 [Member]
Summary of significant accounting policies [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%
Tax Year 2017 [Member]
Summary of significant accounting policies [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		35.00%
The Chubb Corporation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss expenses								$ 22,923
The Chubb Corporation [Member] | Debt Securities [Member]
Summary of significant accounting policies [Line Items]
Assets, Fair Value Adjustment		$ 858			1,652
Selling and Marketing Expense [Member]
Summary of significant accounting policies [Line Items]
Deferred Policy Acquisition Costs, Amortization Expense		116	$ 92				$ 78
Fair Value Adjustment to Acquired Loss Reserves [Member] | The Chubb Corporation [Member]
Summary of significant accounting policies [Line Items]
Increase Decrease on Acquired Unpaid Losses and Loss Expenses		$ 309
Minimum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Interest rates used in calculating future policy benefits		0.01	0.01
Summary of significant accounting policies [Line Items]
Reinsurance Premiums, Amortization Period		1 year
Amortization period for value of reinsurance business assumed		9 years
Finite-Lived Intangible Asset, Useful Life		1 year
Minimum | The Chubb Corporation [Member]
Summary of significant accounting policies [Line Items]
Amortization Period of Increase Decrease to Acquired Unpaid Losses and Loss Expenses		5 years
Minimum | Fair Value Adjustment to Acquired Loss Reserves [Member] | The Chubb Corporation [Member]
Summary of significant accounting policies [Line Items]
Finite-Lived Intangible Asset, Useful Life		5 years
Maximum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Interest rates used in calculating future policy benefits		0.08	0.080
Summary of significant accounting policies [Line Items]
Reinsurance Premiums, Amortization Period		3 years
Amortization period for value of reinsurance business assumed		40 years
Finite-Lived Intangible Asset, Useful Life		30 years
Maximum | The Chubb Corporation [Member]
Summary of significant accounting policies [Line Items]
Amortization Period of Increase Decrease to Acquired Unpaid Losses and Loss Expenses		17 years
Maximum | Fair Value Adjustment to Acquired Loss Reserves [Member] | The Chubb Corporation [Member]
Summary of significant accounting policies [Line Items]
Finite-Lived Intangible Asset, Useful Life		17 years
Structured settlements
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Deposit assets reflected in Other assets		$ 36	$ 38		38
Unpaid losses and loss expenses		586
Reinsurance recoverables for amounts due from life insurance companies		550
Other Short-duration Insurance Product Line [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss expenses		41	$ 50		$ 50
Initial Application Period Cumulative Effect Transition [Domain] | purchased callable debt [Member]
Summary of significant accounting policies [Line Items]
Debt Instrument, Unamortized Premium		$ 30

[1] Chubb maintains two notional multicurrency cash pools (Pools) with a third-party bank. Refer to Note 1 f) for additional information. At December 31, 2016, the cash balance of one or more entities was negative; however, the overall Pool balances were positive.