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Accounts receivable and allowance for doubtful accounts (Details) - USD ($)
$ in Millions
Mar. 31, 2016
Dec. 31, 2015
Mar. 31, 2015
Receivables [Abstract]      
Receivables past due 90 days or more $ 30.5 $ 27.8 $ 27.7
Allowance for doubtful accounts receivable $ 11.1 $ 9.8 $ 9.4