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Retirement benefits - Defined Benefit Plans - Sensitivity Analysis of Actuarial Assumptions (Detail) $ in Millions	Dec. 31, 2018 USD ($)
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	$ (1.0)
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	1.0
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	(1.0)
United Kingdom [member] | 0.25% increase [member] | Higher/(lower) pension cost [member] | Pensions increases [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	0.1
United Kingdom [member] | 0.25% increase [member] | Higher/(lower) pension cost [member] | Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	0.1
United Kingdom [member] | 0.25% increase [member] | Higher/(lower) pension cost [member] | Inflation rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	0.1
United Kingdom [member] | 0.25% increase [member] | Increase/ (decrease) In liabilities [member] | Pensions increases [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	1.0
United Kingdom [member] | 0.25% increase [member] | Increase/ (decrease) In liabilities [member] | Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	(1.0)
United Kingdom [member] | 0.25% increase [member] | Increase/ (decrease) In liabilities [member] | Inflation rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	1.0
United Kingdom [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	0.1
United Kingdom [member] | One year increase [member] | Increase/ (decrease) In liabilities [member] | Mortality rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	0.6
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	1.6
United States [member] | 0.25% increase [member] | Higher/(lower) pension cost [member] | Discount rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase in actuarial assumption	0.1
United States [member] | 0.25% increase [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 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	0.1
United States [member] | One year increase [member] | Increase/ (decrease) In liabilities [member] | Mortality rate [member]
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Decrease in actuarial assumption	$ 3.3

X

- Definition

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+ References

No definition available.

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 145
-Subparagraph a
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