Employee plans - Summary of Changes in Level 3 Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Other Postretirement Benefit Plans
Change in plan assets
Fair value of plan assets at beginning of year	$ 7.9	$ 7.7
Fair value of plan assets at end of year	7.6	7.9
Other Postretirement Benefit Plans \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	7.9
Fair value of plan assets at end of year		7.9
Other Postretirement Benefit Plans \| Level 3 \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	7.9
Fair value of plan assets at end of year		7.9
U.S. Plans \| Pension Plans
Change in plan assets
Fair value of plan assets at beginning of year	3,452.1	3,759.4
Fair value of plan assets at end of year	3,578.4	3,452.1
U.S. Plans \| Pension Plans \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0.0
Fair value of plan assets at end of year	0.0	0.0
U.S. Plans \| Other Postretirement Benefit Plans \| Insurance Contracts
Change in plan assets
Fair value of plan assets at end of year	7.6
U.S. Plans \| Other Postretirement Benefit Plans \| Level 3 \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	7.9	7.7
Realized gains (losses)	(0.2)	(0.3)
Purchases or acquisitions	0.2	0.5
Sales or dispositions	(0.3)	0.0
Currency and unrealized gains (losses) relating to instruments still held at December 31, 2017	0.0	0.0
Fair value of plan assets at end of year	7.6	7.9
International Plans \| Pension Plans
Change in plan assets
Fair value of plan assets at beginning of year	2,429.7	2,496.8
Fair value of plan assets at end of year	2,833.9	2,429.7
International Plans \| Pension Plans \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	116.2
Fair value of plan assets at end of year	135.8	116.2
International Plans \| Pension Plans \| Level 3
Change in plan assets
Fair value of plan assets at beginning of year	116.2
Fair value of plan assets at end of year	135.8	116.2
International Plans \| Pension Plans \| Level 3 \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	116.2	120.6
Realized gains (losses)	0.0	0.0
Purchases or acquisitions	10.8	4.7
Sales or dispositions	(11.4)	(11.0)
Currency and unrealized gains (losses) relating to instruments still held at December 31, 2017	20.2	1.9
Fair value of plan assets at end of year	$ 135.8	$ 116.2

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

+ References

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- Definition

Amount of increase in plan assets of defined benefit plan from business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(6)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of decrease in plan assets of defined benefit plan from disposal or classification as held-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(7)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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