Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2017	Jan. 31, 2016
Revenue from external customers
Total Net Sales	$ 562,523	$ 420,482	$ 1,704,169	$ 1,331,000
Wallboard
Revenue from external customers
Total Net Sales	254,979	196,946	776,250	622,123
Ceilings
Revenue from external customers
Total Net Sales	81,768	65,370	253,518	218,951
Steel framing
Revenue from external customers
Total Net Sales	93,514	65,932	273,931	203,571
Other products
Revenue from external customers
Total Net Sales	$ 132,262	$ 92,234	$ 400,470	$ 286,355