Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended
Jul. 31, 2019	Jul. 31, 2018
Revenue from external customers
Total net sales	$ 847,176	$ 778,144
Wallboard
Revenue from external customers
Total net sales	341,595	317,735
Ceilings
Revenue from external customers
Total net sales	129,110	115,855
Steel framing
Revenue from external customers
Total net sales	131,829	129,112
Other products
Revenue from external customers
Total net sales	$ 244,642	$ 215,442

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

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