Commitment and contingencies - Narrative (Details) barrel in Thousands, T in Thousands, $ in Millions	12 Months Ended
	Oct. 31, 2021 USD ($) T	Jul. 31, 2021 USD ($) a barrel	Oct. 31, 2020 USD ($) T	Jul. 31, 2020 USD ($)	Jul. 31, 2019 USD ($)
Commitments and Contingencies [Line Items]
Area of property (acre) | a		150
Operating lease, rent expense		$ 4.0		$ 4.1	$ 2.3
Long-term purchase contract, amount purchased, mass | T			12
Purchase commitment, quantity | barrel		6
Purchase commitment, remaining amount committed		$ 6.4
COVID-19, incremental costs		0.7		$ 1.4
Forecast
Commitments and Contingencies [Line Items]
Long-term purchase contract, amount purchased, mass | T	32
Euro Member Countries, Euro
Commitments and Contingencies [Line Items]
Purchase commitment, remaining amount committed		$ 4.8
Inventories
Commitments and Contingencies [Line Items]
Long-term purchase commitment, amount purchased, cost			$ 26.5
Inventories | Forecast
Commitments and Contingencies [Line Items]
Long-term purchase commitment, amount purchased, cost	$ 69.0
Minimum
Commitments and Contingencies [Line Items]
Lease, term of contract		2 years
Long-term purchase contract, period		1 year
Maximum
Commitments and Contingencies [Line Items]
Lease, term of contract		30 years
Long-term purchase contract, period		8 years