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Postretirement benefits (Narrative) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Oct. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Expenses recognized during period under the defined contributions plans |  |  | $ 166 | $ 150 | $ 117
Available for sale, Fair Value |  | $ 80,115 | 78,939 | 80,115 |
Defined benefit plan, benefits paid inclusive of settlements |  |  | 200 | 213 |
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract] |  |  |  |  |
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation |  |  | 8 |  |
Effect of One Percentage Point Increase on Service and Interest Cost Components |  |  | 1 |  |
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation |  |  | 7 |  |
Effect of One Percentage Point Decrease on Service and Interest Cost Components |  |  | $ 1 |  |
Minimum |  |  |  |  |
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract] |  |  |  |  |
Defined Benefit Plan, Target Allocation Percentage |  |  | 0.55 |  |
Maximum |  |  |  |  |
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract] |  |  |  |  |
Defined Benefit Plan, Target Allocation Percentage |  |  | 0.65 |  |
Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Accumulated Benefit Obligation |  | 3,800 | $ 4,300 | 3,800 |
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  |  | 28 |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
Defined Benefit Plan, Accumulated Benefit Obligation |  | 3,800 | 4,300 | 3,800 |
Other Postretirement Benefits Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 159 | 157 | 159 | 0
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  |  | 2 |  |
Non-US [Member] | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 962 | 1,172 | 962 | 564
Future Amortization of (Gain) Loss |  |  | 1 |  |
UNITED STATES |  |  |  |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
Effect of Plan Amendment on Accumulated Benefit Obligation |  | 496 |  |  |
UNITED STATES | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 2,765 | 3,109 | 2,765 | $ 9
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
Effect of Plan Amendment on Accumulated Benefit Obligation |  | 113 |  |  |
UNITED STATES | Other Postretirement Benefits Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Future Amortization of Prior Service (Cost) Credit |  |  | 80 |  |
UNITED STATES | Postretirement Health Coverage [Member] |  |  |  |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
Effect of Plan Amendment on Accumulated Benefit Obligation | $ 383 |  |  |  |
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract] |  |  |  |  |
Remaining Balance of Change in Benefit Obligation For Plan Amendment |  |  | 267 |  |
UNITED STATES | Postretirement Health Coverage [Member] | Subject to Amortization [Member] |  |  |  |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
Effect of Plan Amendment on Accumulated Benefit Obligation | $ 410 | 15 | 89 |  |
UNITED STATES | Postretirement Health Coverage [Member] | Subject to Amortization [Member] | The Chubb Corporation [Member] |  |  |  |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
Effect of Plan Amendment on Accumulated Benefit Obligation |  |  | 39 |  |
Foreign Government Debt Securities [Member] | Non-US [Member] | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Available for sale, Fair Value |  | 435 | 456 | 435 |
Foreign Government Debt Securities [Member] | UNITED STATES | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Available for sale, Fair Value |  | 487 | 692 | 487 |
Level 3 | Foreign Government Debt Securities [Member] | Non-US [Member] | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Available for sale, Fair Value |  | 0 | 0 | 0 |
Level 3 | Foreign Government Debt Securities [Member] | UNITED STATES | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Available for sale, Fair Value |  | $ 5 | $ 0 | $ 5 |

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