Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Receivables [Abstract]
Receivables past due 90 days or more	$ 32.7	$ 29.2	$ 31.7
Allowance for doubtful accounts receivable	$ 9.2	$ 10.5	$ 11.0