Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Receivables [Abstract]
Receivables past due 90 days or more	$ 56.0	$ 30.0	$ 29.2
Allowance for doubtful accounts receivable	$ 8.7	$ 8.9	$ 7.3