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Accounts receivable and allowance for doubtful accounts (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Accounting Policies [Abstract]    
Balance of receivables past due 90 days or more $ 36.4 $ 34.3
Allowance for doubtful accounts $ 10.1 $ 10.8