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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 |  |

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