Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2017	Jul. 31, 2016
Revenue from external customers
Total net sales	$ 642,157	$ 549,800
Wallboard
Revenue from external customers
Total net sales	284,657	251,296
Ceilings
Revenue from external customers
Total net sales	99,710	86,349
Steel framing
Revenue from external customers
Total net sales	104,651	84,343
Other products
Revenue from external customers
Total net sales	$ 153,139	$ 127,812