Retirement benefits - Summary of Plan Assets Measured at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Disclosure of defined benefit plans [line items]
Fair value of plan assets	$ 152	$ 148
United States [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	152	148
United States [member] \| Investments quoted in active markets [member]
Disclosure of defined benefit plans [line items]
Investment funds: fixed income securities	150	146
United States [member] \| Unquoted investments [member]
Disclosure of defined benefit plans [line items]
Cash	$ 2	$ 2

The amount that cash and cash equivalents constitute of the fair value of defined benefit plan assets. [Refer: Cash and cash equivalents; Plan assets, at fair value; Defined benefit plans [member]]

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The amount that investment funds constitute of the fair value of defined benefit plan assets. [Refer: Plan assets, at fair value; Defined benefit plans [member]; Investment funds [member]]

+ References

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- Definition

The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

+ References

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