Segments - Net sales by major geographic area (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2020	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales	$ 770,850	$ 761,352	$ 861,929	$ 847,176	$ 780,149	$ 723,902	$ 833,837	$ 778,144	$ 3,241,307	$ 3,116,032	$ 2,511,469
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales									2,805,920	2,701,678	$ 2,511,469
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Total net sales									$ 435,387	$ 414,354