Business Acquisitions (Details) $ in Thousands	6 Months Ended
	Oct. 31, 2017 USD ($) segment	Oct. 31, 2016 USD ($)	Apr. 30, 2017 USD ($)	Sep. 01, 2016
Purchase price
Number of reportable segments | segment	1
Allocation of consideration for acquisitions
Goodwill	$ 426,288		$ 423,644
Adjustments/Reclassifications
Goodwill	115
As previously reported
Adjustments/Reclassifications
Contingent consideration paid out		$ 4,326
2018 Acquisitions
Purchase price
Aggregate purchase price	17,300
Allocation of consideration for acquisitions
Trade accounts and notes receivable	4,250
Inventories	2,814
Property and equipment	794
Goodwill	2,529
Liabilities assumed	(1,485)
Purchase price	17,322
2018 Acquisitions | Tradenames
Allocation of consideration for acquisitions
Definite lived intangible assets	1,000
2018 Acquisitions | Vendor Agreement
Allocation of consideration for acquisitions
Definite lived intangible assets	1,000
2018 Acquisitions | Below market leases
Allocation of consideration for acquisitions
Definite lived intangible assets	350
2018 Acquisitions | Non-compete agreements
Allocation of consideration for acquisitions
Definite lived intangible assets	270
2018 Acquisitions | Customer relationships
Allocation of consideration for acquisitions
Definite lived intangible assets	5,800
2017 Acquisitions
Purchase price
Aggregate purchase price	154,000
Contingent consideration	5,300
Cash consideration	148,700
Allocation of consideration for acquisitions
Cash and cash equivalents	1,558
Trade accounts and notes receivable	37,929
Inventories	16,504
Other current assets	671
Property and equipment	8,357
Goodwill	37,843
Deferred tax liability	(6,011)
Liabilities assumed	(16,958)
Purchase price	154,043
Adjustments/Reclassifications
Trade accounts and notes receivable	238
Other current assets	14
Goodwill	115
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	25,600
Other intangible assets expected to be deductible for U.S. federal income tax purposes	53,600
Goodwill nondeductible for U.S. federal income tax purposes	12,200
Other intangible assets nondeductible for U.S. federal income tax purposes	20,600
2017 Acquisitions | Tradenames
Allocation of consideration for acquisitions
Definite lived intangible assets	9,490
2017 Acquisitions | Customer relationships
Allocation of consideration for acquisitions
Definite lived intangible assets	$ 64,660
2017 Acquisitions | As previously reported
Allocation of consideration for acquisitions
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
Allocation of consideration for acquisitions
Definite lived intangible assets			9,490
2017 Acquisitions | As previously reported | Customer relationships
Allocation of consideration for acquisitions
Definite lived intangible assets			$ 64,660
Olympia Building Supplies, LLC/Redmill, Inc.
Business Acquisitions
Outstanding common stock purchased (as a percent)				100.00%