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Post-retirement benefits - Results summary of sensitivity analysis (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Actuarial assumption of discount rates [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of percentage points | 0.50% | 0.50%
Decrease of percentage points | 0.50% | 0.50%
Actuarial assumption of discount rates [Member] | Other retirement benefits [Member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of percentage points | 0.50% | 0.50%
Decrease of percentage points | 0.50% | 0.50%
Actuarial assumption of expected rates of inflation [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of percentage points | 0.50% | 0.50%
Decrease of percentage points | 0.50% | 0.50%
Actuarial assumption of expected rates of inflation [Member] | Other retirement benefits [Member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of percentage points | 0.50% | 0.50%
Decrease of percentage points | 0.50% | 0.50%
Actuarial assumption of expected rates of salary increases [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of percentage points | 0.50% | 0.50%
Decrease of percentage points | 0.50% | 0.50%
Actuarial assumption of expected rates of salary increases [Member] | Other retirement benefits [Member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of percentage points | 0.50% | 0.50%
Decrease of percentage points | 0.50% | 0.50%
Actuarial assumption of allowance for future improvements in longevity [member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase in actuarial assumption of longevity trend | 1 year | 1 year
Decrease in actuarial assumption of longevity trend | 1 year | 1 year
Actuarial assumption of allowance for future improvements in longevity [member] | Other retirement benefits [Member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase in actuarial assumption of longevity trend | 1 year | 1 year
Decrease in actuarial assumption of longevity trend | 1 year | 1 year
Actuarial assumption of medical cost trend rates [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of percentage points | 1.00% | 1.00%
Decrease of percentage points | 1.00% | 1.00%
Actuarial assumption of medical cost trend rates [Member] | Other retirement benefits [Member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of percentage points | 1.00% | 1.00%
Decrease of percentage points | 1.00% | 1.00%

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