Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Revenue from external customers
Total net sales	$ 648,004	$ 591,846	$ 1,290,161	$ 1,141,646
Wallboard
Revenue from external customers
Total net sales	288,498	269,975	573,155	521,271
Ceilings
Revenue from external customers
Total net sales	101,646	85,400	201,356	171,749
Steel framing
Revenue from external customers
Total net sales	103,203	96,075	207,854	180,417
Other products
Revenue from external customers
Total net sales	$ 154,657	$ 140,396	$ 307,796	$ 268,209