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, 2016	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Revenue from external customers
Total net sales	$ 614,977	$ 562,523	$ 591,846	$ 549,800	$ 527,182	$ 420,482	$ 458,077	$ 452,441	$ 2,319,146	$ 1,858,182	$ 1,570,085
Wallboard
Revenue from external customers
Total net sales									1,058,400	870,952	718,102
Ceilings
Revenue from external customers
Total net sales									341,007	297,110	278,749
Steel framing
Revenue from external customers
Total net sales									374,151	281,340	243,173
Other products
Revenue from external customers
Total net sales									$ 545,588	$ 408,780	$ 330,061