Retirement benefits - Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Disclosure of fair value of plan assets [line items]
Equities and growth funds	$ 226.3	$ 192.2
Government bonds	49.9	44.3
Corporate bonds	91.0	80.5
Cash	0.3	(0.1)
Total market value of assets	367.5	316.9
Present value of plan liabilities	(422.8)	(383.4)
Deficit in the plans	(55.3)	(66.5)
Related deferred income tax assets	10.9	14.6
Net pension liabilities	(44.4)	(51.9)
U.K.
Disclosure of fair value of plan assets [line items]
Equities and growth funds	203.4	171.6
Government bonds	49.9	44.3
Corporate bonds	72.7	64.5
Cash	0.3	(0.1)
Total market value of assets	326.3	280.3
Present value of plan liabilities	(369.7)	(334.8)
Deficit in the plans	(43.4)	(54.5)
Related deferred income tax assets	7.8	10.2
Net pension liabilities	(35.6)	(44.3)
Non-U.K.
Disclosure of fair value of plan assets [line items]
Equities and growth funds	22.9	20.6
Government bonds	0.0	0.0
Corporate bonds	18.3	16.0
Cash	0.0	0.0
Total market value of assets	41.2	36.6
Present value of plan liabilities	(53.1)	(48.6)
Deficit in the plans	(11.9)	(12.0)
Related deferred income tax assets	3.1	4.4
Net pension liabilities	$ (8.8)	$ (7.6)

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 present value, without deducting any plan assets, of expected future payments required to settle the obligation resulting from employee service in the current and prior periods. [Refer: Plan assets, at fair value]

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The amount that instruments representing equity (rather than debt) constitute of the fair value of defined benefit plan assets. [Refer: Plan assets, at fair value; Defined benefit plans [member]]

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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]]

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The amount of deficit in a defined benefit plan. [Refer: Defined benefit plans [member]]

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The present value of the defined benefit obligation, less the fair value of the plan assets (if any). [Refer: Plan assets [member]]

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Corporate Bonds, Amount Contributed To Fair Value Of Plan Assets

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Government Bonds, Amount Contributed To Fair Value Of Plan Assets

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