Accounts receivable - Past Due Status (Details) $ in Millions	Oct. 31, 2020 USD ($)
Financing Receivable, Past Due [Line Items]
Current	$ 47.6
Total at amortized cost	54.2
1 - 30 days past due
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	4.0
31 - 60 days past due
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	1.5
61 - 90 days past due
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	0.7
Greater than 90 days past due
Financing Receivable, Past Due [Line Items]
Financing receivable, past due	$ 0.4