Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2016	Jul. 31, 2015
Revenue from external customers
Total Net Sales	$ 549,800	$ 452,441
Wallboard
Revenue from external customers
Total Net Sales	$ 251,296	$ 210,922
Wallboard | Sales Revenue, Product Line | Product Concentration Risk
Revenue from external customers
Percentage of sales	45.70%	46.60%
Ceilings
Revenue from external customers
Total Net Sales	$ 86,349	$ 78,967
Ceilings | Sales Revenue, Product Line | Product Concentration Risk
Revenue from external customers
Percentage of sales	15.70%	17.50%
Steel Framing
Revenue from external customers
Total Net Sales	$ 84,343	$ 67,332
Steel Framing | Sales Revenue, Product Line | Product Concentration Risk
Revenue from external customers
Percentage of sales	15.30%	14.90%
Other Products
Revenue from external customers
Total Net Sales	$ 127,812	$ 95,220
Other Products | Sales Revenue, Product Line | Product Concentration Risk
Revenue from external customers
Percentage of sales	23.30%	21.00%