Segments - Net Sales by Main Product Lines (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
Revenue from external customers
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
Wallboard
Revenue from external customers
Total net sales									1,329,775	1,272,068	1,109,552
Ceilings
Revenue from external customers
Total net sales									475,827	451,695	387,360
Steel framing
Revenue from external customers
Total net sales									502,122	506,805	411,630
Other products
Revenue from external customers
Total net sales									$ 933,583	$ 885,464	$ 602,927