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Post-retirement benefits - Results summary of sensitivity analysis (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
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

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- Definition The reasonably possible percentage of the decrease in the actuarial assumption used to determine the present value of defined benefit obligation. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_145_a&doctype=Standard -URIDate 2017-03-09 + Details
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