Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Receivables [Abstract]
Receivables past due 90 days or more	$ 26.3	$ 27.8	$ 29.3
Allowance for doubtful accounts receivable	$ 10.2	$ 9.8	$ 9.0