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Post-retirement benefits - Summary of geographical distribution of defined benefit obligations (Detail) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 100.00% | 100.00% | 100.00%
Pension benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 100.00% |  |
Other Benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 100.00% |  |
Canada [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 49.00% | 46.00% | 44.00%
Canada [member] | Pension benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 49.00% |  |
Canada [member] | Other Benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 41.00% |  |
United Kingdom [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 27.00% | 27.00% | 29.00%
United Kingdom [member] | Pension benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 29.00% |  |
United Kingdom [member] | Other Benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 2.00% |  |
United States [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 14.00% | 16.00% | 15.00%
United States [member] | Pension benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 12.00% |  |
United States [member] | Other Benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 54.00% |  |
Switzerland [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 5.00% | 5.00% | 6.00%
Switzerland [member] | Pension benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 5.00% |  |
Switzerland [member] | Other Benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 0.00% |  |
Europe [Member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 3.00% | 4.00% | 4.00%
Europe [Member] | Pension benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 3.00% |  |
Europe [Member] | Other Benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 0.00% |  |
Other countries [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 2.00% | 2.00% | 2.00%
Other countries [member] | Pension benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 2.00% |  |
Other countries [member] | Other Benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Defined benefit obligation | 3.00% |  |

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition Defined benefit obligation geographical distribution. + References No definition available. + Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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