Income taxes - Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Jul. 31, 2021	Jul. 31, 2020	Jul. 31, 2019
Income Tax Disclosure [Abstract]
Federal statutory income tax rate, percent	21.00%	21.00%	21.00%
State income taxes, percent	3.90%	3.30%	2.50%
Stock-based compensation, percent	2.60%	0.00%	0.00%
Transaction costs, percent	0.00%	0.10%	1.90%
Other, percent	(0.20%)	0.00%	0.70%
Effective income tax rate, percent	27.30%	24.40%	26.10%