Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Apr. 30, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Revenue from external customers
Total net sales	$ 614,977	$ 562,523	$ 591,846	$ 549,800	$ 2,511,469	$ 2,319,146	$ 1,858,182
Wallboard
Revenue from external customers
Total net sales					1,109,552	1,058,400	870,952
Ceilings
Revenue from external customers
Total net sales					387,360	341,007	297,110
Steel framing
Revenue from external customers
Total net sales					411,630	374,151	281,340
Other products
Revenue from external customers
Total net sales					$ 602,927	$ 545,588	$ 408,780