Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended
Jul. 31, 2018	Jul. 31, 2017
Revenue from external customers
Total net sales	$ 778,144	$ 642,157
Wallboard
Revenue from external customers
Total net sales	317,735	284,657
Ceilings
Revenue from external customers
Total net sales	115,855	99,710
Steel framing
Revenue from external customers
Total net sales	129,112	104,651
Other products
Revenue from external customers
Total net sales	$ 215,442	$ 153,139

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration