Employee plans - Additional Information About Plan Assets Valued Using Net Asset Value (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
U.S. Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 164.8	$ 150.5
Unfunded Commitments	0.0	0.0
International Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	1,434.2	1,631.3
Unfunded Commitments	0.0	0.0
Equity Securities, Commingled Funds \| U.S. Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	0.0	0.0
Equity Securities, Commingled Funds \| International Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	726.7	880.8
Unfunded Commitments	$ 0.0	0.0
Redemption Notice Period Range	90 days
Debt Securities, Commingled Funds \| U.S. Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 18.6	0.0
Unfunded Commitments	$ 0.0	0.0
Redemption Notice Period Range	5 days
Debt Securities, Commingled Funds \| International Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 640.0	632.6
Unfunded Commitments	$ 0.0	0.0
Redemption Notice Period Range	90 days
Other, Commingled Funds \| U.S. Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 104.6	105.3
Unfunded Commitments	$ 0.0	0.0
Redemption Notice Period Range	5 days
Other, Commingled Funds \| International Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 25.8	76.1
Unfunded Commitments	$ 0.0	0.0
Redemption Notice Period Range	90 days
Private Real Estate \| U.S. Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 40.5	37.6
Unfunded Commitments	$ 0.0	0.0
Redemption Notice Period Range	60 days
Limited partnerships, period expected to make all distributions	3 years
Private Real Estate \| International Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 41.7	41.8
Unfunded Commitments	$ 0.0	0.0
Redemption Notice Period Range	90 days
Private Equity \| U.S. Plans
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 1.1	7.6
Unfunded Commitments	$ 0.0	0.0
Limited partnerships, period expected to make all distributions	3 years
Private Equity \| International Plans
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.

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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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