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Accounts receivable and allowance for doubtful accounts (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Accounting Policies [Abstract]    
Balance of receivables past due 90 days or more $ 30.9us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue $ 36.4us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
Allowance for doubtful accounts $ 9.5us-gaap_AllowanceForDoubtfulAccountsReceivable $ 10.1us-gaap_AllowanceForDoubtfulAccountsReceivable