Business Acquisitions - Aggregate Purchase Price and Allocation of Consideration (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jul. 31, 2016	Apr. 30, 2016	Jan. 01, 2016	Apr. 30, 2015
Purchase price
Net cash considerations	$ 23,278
Allocation of consideration for acquisitions
Goodwill	393,640	$ 386,306		$ 348,811
Adjustments/Reclassifications
Goodwill	(256)	(1,338)
2017 Acquisitions
Purchase price
Aggregate purchase price	25,989
Allocation of consideration for acquisitions
Trade accounts and notes receivable	5,220
Inventories	3,163
Property and equipment	1,697
Other assets	1
Goodwill	7,590
Liabilities assumed	(2,882)
Purchase price	25,989
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes	7,590
Other intangible assets expected to be deductible for U.S. federal income tax purposes	11,200
2017 Acquisitions | Tradenames
Allocation of consideration for acquisitions
Definite lived intangible assets	400
2017 Acquisitions | Customer relationships
Allocation of consideration for acquisitions
Definite lived intangible assets	10,800
2016 Acquisitions
Purchase price
Aggregate purchase price		117,032
Net cash considerations		112,778
Contingent consideration		4,254
Allocation of consideration for acquisitions
Trade accounts and notes receivable	26,809
Inventories	17,543
Property and equipment	9,236
Other assets	1,765
Goodwill	38,577
Deferred tax liability	(6,676)
Liabilities assumed	(13,797)
Purchase price	117,032
Adjustments/Reclassifications
Trade accounts and notes receivable	102
Other assets	1
Goodwill	(256)
Liabilities assumed	7
Purchase price	(146)
Decrease in total consideration paid	146
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes	13,515
Other intangible assets expected to be deductible for U.S. federal income tax purposes	26,335
Goodwill nondeductible for U.S. federal income tax purposes	25,062
Other intangible assets nondeductible for U.S. federal income tax purposes	17,240
2016 Acquisitions | Tradenames
Allocation of consideration for acquisitions
Definite lived intangible assets	12,500
2016 Acquisitions | Below market leases
Allocation of consideration for acquisitions
Definite lived intangible assets	2,020
2016 Acquisitions | Customer relationships
Allocation of consideration for acquisitions
Definite lived intangible assets	$ 29,055
2016 Acquisitions | As previously reported
Allocation of consideration for acquisitions
Trade accounts and notes receivable		26,707
Inventories		17,543
Property and equipment		9,236
Other assets		1,764
Goodwill		38,833
Deferred tax liability		(6,676)
Liabilities assumed		(13,804)
Purchase price		117,178
2016 Acquisitions | As previously reported | Tradenames
Allocation of consideration for acquisitions
Definite lived intangible assets		12,500
2016 Acquisitions | As previously reported | Below market leases
Allocation of consideration for acquisitions
Definite lived intangible assets		2,020
2016 Acquisitions | As previously reported | Customer relationships
Allocation of consideration for acquisitions
Definite lived intangible assets		$ 29,055
Gypsum Supply Company
Business Acquisitions
Outstanding common stock purchased (as a percent)			100.00%