Summary of significant accounting policies (Narrative) (Detail) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
		Jan. 14, 2016	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Deposit Assets				$ 93	$ 97
Unpaid losses and loss expenses			$ 63,179	62,690	62,960	$ 63,179	$ 60,540
Deposit liabilities included in Deposit liabilities				88	97
Borrowings under Guaranteed Investment Agreements				2,000	1,800
Summary of significant accounting policies [Line Items]
Deferred Policy Acquisition Costs, Amortization Expense				6,153	5,912	5,781
Deferred Advertising Costs				$ 246	255
Deferred Marketing Costs, Amortization Period				10 years
Recoverable from unrated reinsurers, ceded reserve, default factor (percent)				34.00%
Reinsurance business assumed				$ 6	14
Percentage of fair value of loaned securities				102.00%
Quality assessment threshold used in goodwill impairment testing				50.00%
Affiliated notional cash pooling program	[1]				0
Net operating results of ESIS included within Administrative expenses				$ 47	49	38
Property, Plant and Equipment, Net				1,900	1,700
Chubb integration expenses				23	59	$ 310
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount					$ 475
Operating Lease, Right-of-Use Asset				551
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%
The Chubb Corporation [Member] | Debt Securities [Member]
Summary of significant accounting policies [Line Items]
Assets, Fair Value Adjustment		$ 1,652		332
Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect on Retained Earnings, before Tax				70
Cumulative Effect on Retained Earnings, Net of Tax				64
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				12
Selling and Marketing Expense [Member]
Summary of significant accounting policies [Line Items]
Deferred Policy Acquisition Costs, Amortization Expense				109	$ 114	$ 116
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				145	207
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			296	0		296	0
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			(264)		12	(264)	$ 0
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			0	0	(47)	0
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			0	$ 0	$ 22	$ 0
Minimum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for Future Policy Benefits, Interest Rate				1.00%	1.00%
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]
Liability for Future Policy Benefits, Interest Rate				11.00%	11.00%
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				25 years
Affiliated notional cash pooling program				$ 300
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
Software Development [Member]
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Net				$ 1,100
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				$ 270
Building [Member] | Maximum
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Useful Life				40 years
Structured settlements
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Deposit Assets				$ 31	33
Unpaid losses and loss expenses				567
Reinsurance recoverables for amounts due from life insurance companies				536
Short-Duration Insurance, Other [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss expenses				$ 43	$ 40

[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, 2018, the cash balance of one or more entities was negative; however, the overall Pool balances were positive.