Transactions With Related Parties - Purchased Inventories (Details) - Southern Wall Products, Inc. - Inventory purchases - USD ($) $ in Millions	12 Months Ended
Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Transactions with related parties
Purchases from related party	$ 14.0	$ 13.0	$ 12.8
Accounts payable
Transactions with related parties
Due to SWP	$ 1.2	$ 1.1

Amount of payable due to an entity that is affiliated with the reporting entity by means of direct or indirect ownership. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(12)(b)(3))
-URI http://asc.fasb.org/extlink&oid=6488278&loc=d3e603758-122996

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(5))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Name:	us-gaap_DueToAffiliateCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant