Summary of significant accounting policies (Narrative) (Detail) - USD ($) $ in Millions				3 Months Ended		12 Months Ended
	Dec. 31, 2017		Jan. 14, 2016	Dec. 31, 2017		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Jan. 01, 2019	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Deposit assets reflected in Other assets	$ 89			$ 89		$ 97		$ 89
Unpaid losses and loss expenses	63,179			63,179		62,960		63,179		$ 60,540		$ 37,303
Deposit liabilities included in Deposit liabilities	100			100		97		100
Borrowings under Guaranteed Investment Agreements	1,800			1,800				1,800
Summary of significant accounting policies [Line Items]
Deferred Policy Acquisition Costs, Amortization Expense						5,912		5,781		5,904
Deferred Advertising Costs	271			271		$ 255		271
Deferred Marketing Costs, Amortization Period						10 years
Recoverable from unrated reinsurers, ceded reserve, default factor (percent)						34.00%
Reinsurance business assumed						$ 14		18
Percentage of fair value of loaned securities						102.00%
Quality assessment threshold used in goodwill impairment testing						50.00%
Affiliated notional cash pooling program	0	[1]		0	[1]	$ 0	[2]	0	[1]
Net operating results of ESIS included within Administrative expenses						$ 49		38		32
Income Tax Examination, Description						0.5
Property, Plant and Equipment, Net	1,300			1,300		$ 1,700		1,300
Chubb integration expenses						$ 59		$ 310		492
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount				475
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%
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%		35.00%
The Chubb Corporation [Member] | Debt Securities [Member]
Summary of significant accounting policies [Line Items]
Assets, Fair Value Adjustment			$ 1,652			$ 520
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect on Retained Earnings, Tax	7
Accounting Standards Update 2017-08 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect on Retained Earnings, before Tax						15
Cumulative Effect on Retained Earnings, Net of Tax						11
Selling and Marketing Expense [Member]
Summary of significant accounting policies [Line Items]
Deferred Policy Acquisition Costs, Amortization Expense						114		$ 116		$ 92
Fair Value Adjustment to Acquired Loss Reserves [Member] | The Chubb Corporation [Member]
Summary of significant accounting policies [Line Items]
Balance of FV adjustment on Unpaid Losses and Loss Expenses	309			309		207		309
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	296			296				296
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(417)			(417)				(417)
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	454
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Accounting standards update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						(121)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(264)			(264)				$ (264)
Retained Earnings | Accounting standards update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						146
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Accounting standards update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						(47)
Accumulated Foreign Currency Adjustment Attributable to Parent [Member] | Accounting standards update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						22
Scenario, Plan [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount				450
Scenario, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount						$ 25
Scenario, Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset											$ 800
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
Maximum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Interest rates used in calculating future policy benefits						0.11		0.08
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
Affiliated notional cash pooling program						$ 300
Software Development [Member]
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Net						$ 970
Property, Plant and Equipment, Useful Life						15 years
Software Development [Member] | Minimum
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Useful Life						3 years
Software Development [Member] | Maximum
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Useful Life						5 years
Building [Member]
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Net						$ 277
Building [Member] | Maximum
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Useful Life						39 years
Structured settlements
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Deposit assets reflected in Other assets	36			36		$ 33		$ 36
Unpaid losses and loss expenses						581
Reinsurance recoverables for amounts due from life insurance companies						548
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			$ 41		$ 40		$ 41

[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, 2017, the cash balance of one or more entities was negative; however, the overall Pool balances were positive.

[2] Chubb maintains two notional multicurrency cash pools (Pools) with a third-party bank. Refer to Note 1 f) for additional information. At December 31, 2018, the cash balance of one or more entities was negative; however, the overall Pool balances were positive.