Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2017
Revenue from external customers
Total net sales	$ 780,149	$ 723,902	$ 833,837	$ 778,144	$ 653,800	$ 585,508	$ 648,004	$ 642,157	$ 3,116,032	$ 2,511,469	$ 2,319,146
Wallboard
Revenue from external customers
Total net sales									1,272,068	1,109,552	1,058,400
Ceilings
Revenue from external customers
Total net sales									451,695	387,360	341,007
Steel framing
Revenue from external customers
Total net sales									506,805	411,630	374,151
Other products
Revenue from external customers
Total net sales									$ 885,464	$ 602,927	$ 545,588