Income taxes - Income and Taxes from Cont. Operations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income (loss) from continuing operations before income taxes [Abstract]
U.S.	$ (79.4)	$ (58.1)	$ (25.3)
Foreign	30.4	197.7	171.4
Income from continuing operations before tax	(49.0)	139.6	146.1
Income tax expense (benefit) from continuing operations [Abstract]
U.S. federal - Current	(3.8)us-gaap_CurrentFederalTaxExpenseBenefit	0.5us-gaap_CurrentFederalTaxExpenseBenefit	(2.0)us-gaap_CurrentFederalTaxExpenseBenefit
State - Current	(0.8)	1.5	(0.3)
Foreign - Current	69.7us-gaap_CurrentForeignTaxExpenseBenefit	81.9us-gaap_CurrentForeignTaxExpenseBenefit	68.8us-gaap_CurrentForeignTaxExpenseBenefit
Current tax expense	65.1us-gaap_CurrentIncomeTaxExpenseBenefit	83.9us-gaap_CurrentIncomeTaxExpenseBenefit	66.5us-gaap_CurrentIncomeTaxExpenseBenefit
U.S. federal - Deferred	(7.6)	(20.6)	(29.9)
State - Deferred	(1.9)	(1.9)	(1.4)
Foreign - Deferred	(18.9)	(12.1)	(12.2)
Deferred income taxes	(28.4)us-gaap_DeferredIncomeTaxExpenseBenefit	(34.6)us-gaap_DeferredIncomeTaxExpenseBenefit	(43.5)us-gaap_DeferredIncomeTaxExpenseBenefit
Provision (benefit) for income taxes	36.7us-gaap_IncomeTaxExpenseBenefit	49.3us-gaap_IncomeTaxExpenseBenefit	23.0us-gaap_IncomeTaxExpenseBenefit
Comprehensive provision (benefit) for income taxes allocation [Abstract]
Provision Continuing Operations	36.7us-gaap_IncomeTaxExpenseBenefit	49.3us-gaap_IncomeTaxExpenseBenefit	23.0us-gaap_IncomeTaxExpenseBenefit
Discontinued operations	0.4	7.4	3.1
Provision (benefit) for income taxes	(43.0)us-gaap_OtherComprehensiveIncomeLossTax	141.0us-gaap_OtherComprehensiveIncomeLossTax	9.3us-gaap_OtherComprehensiveIncomeLossTax
Shareholders' equity	0.6	2.8	2.7
Comprehensive provision (benefit) for income taxes	$ (5.3)	$ 200.5	$ 38.1

Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 6.I.7)

-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Current Tax Expense (or Benefit)

-URI http://asc.fasb.org/extlink&oid=6509736

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph h

-Article 4

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section I

-Paragraph Question 1-7