Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2020	Jan. 31, 2019
Revenue from external customers
Total net sales	$ 761,352	$ 723,902	$ 2,470,457	$ 2,335,883
Wallboard
Revenue from external customers
Total net sales	314,391	297,358	1,006,604	949,781
Ceilings
Revenue from external customers
Total net sales	112,768	105,219	364,685	339,450
Steel framing
Revenue from external customers
Total net sales	118,823	117,432	386,811	382,304
Other products
Revenue from external customers
Total net sales	$ 215,370	$ 203,893	$ 712,357	$ 664,348