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Accounts receivable and allowance for doubtful accounts (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Mar. 31, 2013
Receivables [Abstract]      
Receivables past due 90 days or more $ 22.6 $ 36.4 $ 39.6
Allowance for doubtful accounts receivable $ 10.9 $ 10.1 $ 10.8