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Post-retirement benefits - Summary of fair value of plan assets (Detail) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
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 | 32.40% | 36.80%
Quoted [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 28.60% | 32.50%
Private [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 3.80% | 4.30%
Bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 53.20% | 47.90%
Government fixed income bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 14.30% | 11.30%
Government inflation- linked bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 13.10% | 12.00%
Corporate and other publicly quoted bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 23.50% | 22.30%
Private bonds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 2.30% | 2.30%
Property [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 11.20% | 11.00%
Quoted property funds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 5.80% | 5.80%
Unquoted property funds [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 5.40% | 5.20%
Qualifying Insurance policies [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 0.20% | 0.60%
Cash & other [member] |  |
Disclosure of fair value of plan assets [Line Items] |  |
Proportions of total fair value of assets | 3.00% | 3.70%

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