Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Revenue from external customers
Total net sales	$ 585,508	$ 562,523	$ 1,875,669	$ 1,704,169
Wallboard
Revenue from external customers
Total net sales	256,413	254,979	829,568	776,250
Ceilings
Revenue from external customers
Total net sales	90,360	81,768	291,716	253,518
Steel framing
Revenue from external customers
Total net sales	96,744	93,514	304,598	273,931
Other products
Revenue from external customers
Total net sales	$ 141,991	$ 132,262	$ 449,787	$ 400,470