Segments - Net Sales by Main Product Lines (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Oct. 31, 2019	Oct. 31, 2018	Oct. 31, 2019	Oct. 31, 2018
Revenue from external customers
Total net sales	$ 861,929	$ 833,837	$ 1,709,105	$ 1,611,981
Wallboard
Revenue from external customers
Total net sales	350,618	334,688	692,213	652,423
Ceilings
Revenue from external customers
Total net sales	122,807	118,376	251,917	234,231
Steel framing
Revenue from external customers
Total net sales	136,159	135,760	267,988	264,872
Other products
Revenue from external customers
Total net sales	$ 252,345	$ 245,013	$ 496,987	$ 460,455

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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