Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2021	Jan. 31, 2021	Oct. 31, 2020	Jul. 31, 2020	Apr. 30, 2020	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2021	Apr. 30, 2020	Apr. 30, 2019
Revenue from External Customer [Line Items]
Total net sales	$ 932,203	$ 751,191	$ 812,856	$ 802,573	$ 770,850	$ 761,352	$ 861,929	$ 847,176	$ 3,298,823	$ 3,241,307	$ 3,116,032
Wallboard
Revenue from External Customer [Line Items]
Total net sales									1,346,560	1,329,775	1,272,068
Ceilings
Revenue from External Customer [Line Items]
Total net sales									450,524	475,827	451,695
Steel framing
Revenue from External Customer [Line Items]
Total net sales									469,002	502,122	506,805
Complementary products
Revenue from External Customer [Line Items]
Total net sales									$ 1,032,737	$ 933,583	$ 885,464