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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.

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