Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2016	Oct. 31, 2015	Oct. 31, 2016	Oct. 31, 2015
Revenue from external customers
Total Net Sales	$ 591,846	$ 458,077	$ 1,141,646	$ 910,518
Wallboard
Revenue from external customers
Total Net Sales	269,975	214,254	521,271	425,177
Ceilings
Revenue from external customers
Total Net Sales	85,400	74,613	171,749	153,581
Steel Framing
Revenue from external customers
Total Net Sales	96,075	70,307	180,417	137,639
Other Products
Revenue from external customers
Total Net Sales	$ 140,396	$ 98,903	$ 268,209	$ 194,121