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Post-retirement benefits - Summary of fair value of plan assets (Detail) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 100.00% | 100.00%
Equities [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 20.70% | 32.40%
Quoted [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 16.70% | 28.60%
Private [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 4.00% | 3.80%
Bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 63.60% | 53.20%
Government fixed income bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 18.00% | 14.30%
Government inflation- linked bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 15.50% | 13.10%
Corporate and other publicly quoted bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 27.10% | 23.50%
Private bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 3.00% | 2.30%
Property [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 11.00% | 11.20%
Quoted property funds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 5.10% | 5.80%
Unquoted property funds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 5.90% | 5.40%
Qualifying Insurance policies [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 0.10% | 0.20%
Cash & other [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 4.60% | 3.00%

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