Business Acquisitions - Aggregate Purchase Price and Allocation of Consideration (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jan. 31, 2017	Apr. 30, 2016	Sep. 01, 2016	Jan. 01, 2016	Apr. 30, 2015
Allocation of consideration for acquisitions
Goodwill	$ 425,933	$ 386,306			$ 348,811
Adjustments/Reclassifications
Goodwill	(276)	(1,338)
2017 Acquisitions
Purchase price
Aggregate purchase price	151,186
Cash consideration	145,248
Contingent consideration	5,938
Allocation of consideration for acquisitions
Cash and cash equivalents	1,555
Trade accounts and notes receivable	35,681
Inventories	13,961
Property and equipment	8,615
Other assets	613
Goodwill	39,903
Deferred tax liability	(5,898)
Liabilities assumed	(16,044)
Purchase price	151,186
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes	25,795
Other intangible assets expected to be deductible for U.S. federal income tax purposes	52,200
Goodwill nondeductible for U.S. federal income tax purposes	14,108
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	63,310
Olympia Building Supplies, LLC/Redmill, Inc.
Business Acquisitions
Outstanding common stock purchased (as a percent)			100.00%
2016 Acquisitions
Purchase price
Aggregate purchase price		117,233
Cash consideration		113,801
Contingent consideration		3,432
Allocation of consideration for acquisitions
Cash and cash equivalents	956
Trade accounts and notes receivable	27,029
Inventories	17,543
Property and equipment	9,236
Other assets	810
Goodwill	38,557
Deferred tax liability	(6,676)
Liabilities assumed	(13,797)
Purchase price	117,233
Adjustments/Reclassifications
Trade accounts and notes receivable	322
Other assets	1
Goodwill	(276)
Liabilities assumed	7
Purchase price	54
Decrease in total consideration paid	54
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	25,042
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
Cash and cash equivalents		956
Trade accounts and notes receivable		26,707
Inventories		17,543
Property and equipment		9,236
Other assets		809
Goodwill		38,833
Deferred tax liability		(6,676)
Liabilities assumed		(13,804)
Purchase price		117,179
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%