Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Receivables [Abstract]
Receivables past due 90 days or more	$ 47.2	$ 30.0	$ 36.6
Allowance for doubtful accounts receivable	$ 8.2	$ 8.9	$ 7.8