Purchase commitments (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Term of unrecorded unconditional purchase obligation	one to 46 years
Purchase commitments due
2015	$ 694.7
2016	304.6
2017	161.0
2018	91.0
2019	86.4
Thereafter	910.6
Amounts purchased under various commitments	$ 925.2	$ 861.8	$ 718.4

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- Definition

Describes the term of the unconditional purchase obligation that is not recorded as a liability on the balance sheet.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 440

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25336-109308

+ Details

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- Definition

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References