Employee plans - Additional Information About Plan Assets Valued Using Net Asset Value (Details) - Pension Plans - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
U.S. Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 238.2	$ 164.8
Unfunded Commit-ments	0.0	0.0
U.S. Plans \| Equity Securities, Commingled Funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 0.0	0.0
Redemption Notice Period Range	90 days
U.S. Plans \| Debt Securities, Commingled Funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 19.8	18.6
Unfunded Commit-ments	$ 0.0	$ 0.0
Redemption Notice Period Range	5 days	5 days
U.S. Plans \| Other, Commingled Funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 105.1	$ 104.6
Unfunded Commit-ments	$ 0.0	$ 0.0
Redemption Notice Period Range	5 days	5 days
U.S. Plans \| Private Real Estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 112.4	$ 40.5
Unfunded Commit-ments	$ 0.0	$ 0.0
Redemption Notice Period Range		60 days
Limited partnerships, period expected to make all distributions	3 years
U.S. Plans \| Private Equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 0.9	$ 1.1
Unfunded Commit-ments	$ 0.0	0.0
Limited partnerships, period expected to make all distributions	3 years
International Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 1,660.1	1,434.2
Unfunded Commit-ments	0.0	0.0
International Plans \| Equity Securities, Commingled Funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	780.7	726.7
Unfunded Commit-ments	$ 0.0	$ 0.0
Redemption Notice Period Range	30 days	90 days
International Plans \| Debt Securities, Commingled Funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 837.7	$ 640.0
Unfunded Commit-ments	$ 0.0	$ 0.0
Redemption Notice Period Range	30 days	90 days
International Plans \| Other, Commingled Funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 25.2	$ 25.8
Unfunded Commit-ments	$ 0.0	$ 0.0
Redemption Notice Period Range	30 days	90 days
International Plans \| Private Real Estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 16.5	$ 41.7
Unfunded Commit-ments	$ 0.0	$ 0.0
Redemption Notice Period Range	90 days	90 days
International Plans \| Private Equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 0.0	$ 0.0

Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets

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- Definition

Distributions Per Limited Partnership, Expected Distribution Period

+ References

No definition available.

+ Details

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- Definition

Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and alternative investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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