Stockholders' equity - Schedule of Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ 851.3	$ 818.3	$ 839.8	$ 754.3
Balance as adjusted				754.3
Ending Balance	868.0	853.6	868.0	853.6
Currency Translation
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(245.1)	(201.2)	(245.0)	(211.9)
Other comprehensive income (loss)	7.1	(25.2)	7.0	(14.5)
Ending Balance	(238.0)	(226.4)	(238.0)	(226.4)
Marketable Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance				4.8
Change in accounting principle		(4.8)		(4.8)
Accumulated Defined Benefit Plans Adjustment \| Defined Benefit Pension Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(177.7)	(170.4)	(180.0)	(172.8)
Other comprehensive income (loss)	2.2	2.5	4.5	4.9
Ending Balance	(175.5)	(167.9)	(175.5)	(167.9)
Accumulated Defined Benefit Plans Adjustment \| OPEB Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	0.6	1.1	0.7	1.2
Other comprehensive income (loss)		(0.1)	(0.1)	(0.2)
Ending Balance	0.6	1.0	0.6	1.0
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(422.2)	(370.5)	(424.3)	(378.7)
Change in accounting principle		(4.8)		(4.8)	$ (4.8)
Balance as adjusted	(422.2)	(370.5)	(424.3)	(383.5)
Other comprehensive income (loss)	9.3	(22.8)	11.4	(9.8)
Ending Balance	$ (412.9)	$ (393.3)	$ (412.9)	$ (393.3)

Stockholders equity with cumulative effect of new accounting principle.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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