Retirement benefits - Defined Benefit Plans - Sensitivity Analysis of Actuarial Assumptions (Detail) $ in Millions	Dec. 31, 2017 USD ($)
United Kingdom [member] \| 0.25% decrease [member] \| Higher/(lower) pension cost [member] \| Pensions increases [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	$ (0.1)
United Kingdom [member] \| 0.25% decrease [member] \| Higher/(lower) pension cost [member] \| Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	(0.1)
United Kingdom [member] \| 0.25% decrease [member] \| Higher/(lower) pension cost [member] \| Inflation rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	(0.1)
United Kingdom [member] \| 0.25% decrease [member] \| Increase/ (decrease) In liabilities [member] \| Pensions increases [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	(1.1)
United Kingdom [member] \| 0.25% decrease [member] \| Increase/ (decrease) In liabilities [member] \| Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	1.5
United Kingdom [member] \| 0.25% decrease [member] \| Increase/ (decrease) In liabilities [member] \| Inflation rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	(1.1)
United Kingdom [member] \| 0.25% increase [member] \| Increase/ (decrease) In liabilities [member] \| Pensions increases [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	1.4
United Kingdom [member] \| 0.25% increase [member] \| Increase/ (decrease) In liabilities [member] \| Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	(1.4)
United Kingdom [member] \| 0.25% increase [member] \| Increase/ (decrease) In liabilities [member] \| Inflation rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	1.4
United Kingdom [member] \| One year increase [member] \| Increase/ (decrease) In liabilities [member] \| Mortality rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	0.8
United States [member] \| 0.25% decrease [member] \| Higher/(lower) pension cost [member] \| Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	(0.1)
United States [member] \| 0.25% decrease [member] \| Increase/ (decrease) In liabilities [member] \| Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	2.9
United States [member] \| 0.25% increase [member] \| Increase/ (decrease) In liabilities [member] \| Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	(2.7)
United States [member] \| One year increase [member] \| Higher/(lower) pension cost [member] \| Mortality rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	0.2
United States [member] \| One year increase [member] \| Increase/ (decrease) In liabilities [member] \| Mortality rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	$ 9.7

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.

No definition available.

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- Definition

The increase (decrease) in a defined benefit obligation that would have been caused by a decrease in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [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
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-URIDate 2017-03-09

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- Definition

The increase (decrease) in a defined benefit obligation that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]]

+ References

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