Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2018	Oct. 31, 2017
Revenue from external customers
Total net sales	$ 833,837	$ 648,004	$ 1,611,981	$ 1,290,161
Wallboard
Revenue from external customers
Total net sales	334,688	288,498	652,423	573,155
Ceilings
Revenue from external customers
Total net sales	118,376	101,646	234,231	201,356
Steel framing
Revenue from external customers
Total net sales	135,760	103,203	264,872	207,854
Other products
Revenue from external customers
Total net sales	$ 245,013	$ 154,657	$ 460,455	$ 307,796