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Post-retirement benefits - Additional information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of defined benefit plans [Line Items]
Securities included in fair value of plan assets	$ 4,000,000	$ 3,000,000
Defined benefit expenses excluded form unerlying earnings	0	0	$ 0
Past service cost	(2,000,000)	$ 0	(36,000,000)
Excess of service cost over cash paid to defined contribution plans	3,000,000
Estimated contributions to defined benefit pension plans for 2019	$ 150,000,000
Defined benefit obligation weighted average expected future life time	27 years	27 years
Defined benefit obligation weighted average expected future life time at man aged 60 in 2036	28 years	28 years
Healthcare plans
Disclosure of defined benefit plans [Line Items]
Discount rate	2.50%	3.30%
Medical trend rate	6.30%	6.10%
Reduction in medical trend rate	4.60%	4.60%
UK
Disclosure of defined benefit plans [Line Items]
Estimate of the cost of equalising benefits due to court judgement			$ 9,000,000
Past service cost	$ 1,000,000
Discount rate	1.20%	2.00%

X

- Definition

The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

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Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

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- Definition

The fair value of the entity's own transferable financial instruments that are included in the fair value of defined benefit plan assets. [Refer: Plan assets, at fair value; Financial instruments, class [member]; Defined benefit plans [member]]

+ References

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The estimate of the expected contributions to be made to a defined benefit plan for the next annual reporting period. [Refer: Defined benefit plans [member]]

+ References

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The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost. Past service cost is the change in the present value of the defined benefit obligation for employee service in prior periods, resulting from a plan amendment (the introduction or withdrawal of, or changes to, a defined benefit plan) or a curtailment (a significant reduction by the entity in the number of employees covered by a plan). [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

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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]]

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Current estimate of cost of equalising benefits in group's Uk scheme.

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No definition available.

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Defined benefit expenses exlcuded from underlying earnings.

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Defined benefit obligation weighted average expected future life After Balance Sheet Date

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Defined benefit obligation weighted average expected future life time at balance sheet date.

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Excess of service cost over cash paid to defined contribution plans

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