Employee plans - Summary of Changes in Level 3 Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
U.S. Plans
Change in plan assets
Fair value of plan assets at beginning of year	$ 3,759.4	$ 4,069.7
Fair value of plan assets at end of year	3,452.1	3,759.4
Other Postretirement Benefit Plans
Change in plan assets
Fair value of plan assets at beginning of year	7.7	9.1
Fair value of plan assets at end of year	7.9	7.7
Other Postretirement Benefit Plans \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	7.7
Fair value of plan assets at end of year	7.9	7.7
International Plans
Change in plan assets
Fair value of plan assets at beginning of year	2,496.8	2,718.9
Fair value of plan assets at end of year	2,429.7	2,496.8
International Plans \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	120.6
Fair value of plan assets at end of year	116.2	120.6
Level 3 \| U.S. Plans
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
Level 3 \| U.S. Plans \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	0.0	17.4
Realized gains (losses)		(0.4)
Purchases or acquisitions		0.0
Sales or dispositions		(16.6)
Currency and unrealized gains (losses) relating to instruments still held at December 31, 2016		(0.4)
Fair value of plan assets at end of year		0.0
Level 3 \| Other Postretirement Benefit Plans \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	7.7	7.3
Realized gains (losses)	(0.3)	(0.1)
Purchases or acquisitions	0.5	0.5
Sales or dispositions	0.0	0.0
Currency and unrealized gains (losses) relating to instruments still held at December 31, 2016	0.0	0.0
Fair value of plan assets at end of year	7.9	7.7
Level 3 \| International Plans
Change in plan assets
Fair value of plan assets at beginning of year	120.6
Fair value of plan assets at end of year	116.2	120.6
Level 3 \| International Plans \| Insurance Contracts
Change in plan assets
Fair value of plan assets at beginning of year	120.6	135.5
Realized gains (losses)	0.0	0.0
Purchases or acquisitions	4.7	9.4
Sales or dispositions	(11.0)	(10.9)
Currency and unrealized gains (losses) relating to instruments still held at December 31, 2016	1.9	(13.4)
Fair value of plan assets at end of year	$ 116.2	$ 120.6

Actual return on plan assets, separately identifying the amount related to assets sold during the period.

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)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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- Definition

Actual return on plan assets, separately identifying the amount related to assets still held at the reporting date.

+ References

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- Definition

The amount of increase in the plan assets attributed to a business combination.

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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.

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- Definition

The amount of decrease in the plan assets attributed to the sale, liquidation, spin-off or other divestiture of a corporate division or subsidiary.

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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 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930

Reference 2: 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=65897934&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6749337

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6520858

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