Business Acquisitions (Details) $ in Thousands	12 Months Ended
	Apr. 30, 2018 USD ($) segment	Apr. 30, 2017 USD ($)	Apr. 30, 2016 USD ($)	Sep. 01, 2016	Jan. 01, 2016
Business Acquisitions
Number of reportable segments | segment	1
Preliminary Purchase Price Allocation
Goodwill	$ 427,645	$ 423,644	$ 386,306
Adjustments/Reclassifications
Goodwill	(144)	(390)
2018 Acquisitions
Purchase price
Aggregate purchase price	24,400
Preliminary Purchase Price Allocation
Trade accounts and notes receivable	4,872
Inventories	4,321
Property and equipment	1,081
Goodwill	4,145
Liabilities assumed	(1,951)
Purchase price	24,446
2018 Acquisitions | Tradenames
Preliminary Purchase Price Allocation
Definite lived intangible assets	1,000
2018 Acquisitions | Vendor agreement
Preliminary Purchase Price Allocation
Definite lived intangible assets	1,000
2018 Acquisitions | Other intangible assets
Preliminary Purchase Price Allocation
Definite lived intangible assets	620
2018 Acquisitions | Customer relationships
Preliminary Purchase Price Allocation
Definite lived intangible assets	9,358
2017 Acquisitions
Purchase price
Aggregate purchase price		154,000
Cash consideration		148,700
Contingent consideration		5,300
Preliminary Purchase Price Allocation
Cash and cash equivalents	1,558
Trade accounts and notes receivable	37,628
Inventories	16,504
Other current assets	671
Property and equipment	8,357
Goodwill	37,584
Deferred tax liability	(6,011)
Liabilities assumed	(16,398)
Purchase price	154,043
Adjustments/Reclassifications
Trade accounts and notes receivable	(63)
Other current assets	14
Goodwill	(144)
Liabilities assumed	560
Purchase price	367
Increase in total consideration paid	400
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes	25,400
Other intangible assets expected to be deductible for U.S. federal income tax purposes	53,600
Goodwill expected to be nondeductible for U.S. federal income tax purposes	12,200
Other intangible assets expected to be nondeductible for U.S. federal income tax purposes	20,600
2017 Acquisitions | Tradenames
Preliminary Purchase Price Allocation
Definite lived intangible assets	9,490
2017 Acquisitions | Customer relationships
Preliminary Purchase Price Allocation
Definite lived intangible assets	$ 64,660
2017 Acquisitions | As previously reported
Preliminary Purchase Price Allocation
Cash and cash equivalents		1,558
Trade accounts and notes receivable		37,691
Inventories		16,504
Other current assets		657
Property and equipment		8,357
Goodwill		37,728
Deferred tax liability		(6,011)
Liabilities assumed		(16,958)
Purchase price		153,676
2017 Acquisitions | As previously reported | Tradenames
Preliminary Purchase Price Allocation
Definite lived intangible assets		9,490
2017 Acquisitions | As previously reported | Customer relationships
Preliminary Purchase Price Allocation
Definite lived intangible assets		$ 64,660
Olympia Building Supplies, LLC/Redmill, Inc.
Business Acquisitions
Outstanding common stock purchased (as a percent)				100.00%
2016 Acquisitions
Purchase price
Aggregate purchase price			117,200
Cash consideration			114,800
Contingent consideration			2,400
Preliminary Purchase Price Allocation
Cash and cash equivalents			953
Trade accounts and notes receivable			26,988
Inventories			17,703
Property and equipment			9,236
Other assets			808
Goodwill			38,399
Deferred tax liability			(6,676)
Liabilities assumed			(13,805)
Purchase price			117,181
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes			13,400
Other intangible assets expected to be deductible for U.S. federal income tax purposes			26,300
Goodwill expected to be nondeductible for U.S. federal income tax purposes			25,000
Other intangible assets expected to be nondeductible for U.S. federal income tax purposes			17,200
2016 Acquisitions | Tradenames
Preliminary Purchase Price Allocation
Definite lived intangible assets			12,500
2016 Acquisitions | Below market leases
Preliminary Purchase Price Allocation
Definite lived intangible assets			2,020
2016 Acquisitions | Customer relationships
Preliminary Purchase Price Allocation
Definite lived intangible assets			$ 29,055
Gypsum Supply Company
Business Acquisitions
Outstanding common stock purchased (as a percent)					100.00%