Summary of significant accounting policies (Narrative) (Detail) - USD ($) $ in Millions		12 Months Ended
	Jan. 14, 2016	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Deposit Assets		$ 107	$ 93
Unpaid losses and loss expenses		67,811	62,690	$ 62,960	$ 63,179
Deposit liabilities included in Deposit liabilities		86	88
Borrowings under Guaranteed Investment Agreements		2,200	2,000
Summary of significant accounting policies [Line Items]
Deferred Policy Acquisition Costs, Amortization Expense		6,547	6,153	5,912
Deferred Advertising Costs		$ 226	246
Deferred Marketing Costs, Amortization Period		10 years
Recoverable from unrated reinsurers, ceded reserve, default factor (percent)		34.00%
Percentage of fair value of loaned securities		102.00%
Quality assessment threshold used in goodwill impairment testing		50.00%
Net operating results of ESIS included within Administrative expenses		$ 18	47	49
Property, Plant and Equipment, Net		2,000	1,900
Chubb integration expenses		0	23	59
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent		(59,441)	(55,331)	(50,312)
The Chubb Corporation [Member] | Debt Securities [Member]
Summary of significant accounting policies [Line Items]
Assets, Fair Value Adjustment	$ 1,652	213
Selling and Marketing Expense [Member]
Summary of significant accounting policies [Line Items]
Deferred Policy Acquisition Costs, Amortization Expense		99	109	114
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		110	145
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent		$ (39,337)	$ (36,142)	(31,700)	(27,474)
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
Maximum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for Future Policy Benefits, Interest Rate		9.00%	11.00%
Summary of significant accounting policies [Line Items]
Reinsurance Premiums, Amortization Period		3 years
Finite-Lived Intangible Asset, Useful Life		25 years
Affiliated notional cash pooling program		$ 300
Structured settlements
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Deposit Assets		26	$ 31
Unpaid losses and loss expenses		548
Reinsurance recoverables for amounts due from life insurance companies		522
Short-Duration Insurance, Other [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss expenses		42	43
Software Development [Member]
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Net		$ 1,300
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		$ 260
Building [Member] | Maximum
Summary of significant accounting policies [Line Items]
Property, Plant and Equipment, Useful Life		40 years
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent			72	$ 12	$ (264)
Cumulative effect on retained earnings, before tax [Member] | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent			$ 79