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Accounts receivable and allowance for doubtful accounts (Details) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2012
Receivables [Abstract]      
Receivables past due 90 days or more $ 31.1 $ 34.3 $ 35.1
Allowance for doubtful accounts receivable $ 9.6 $ 10.8 $ 10.5