Business Acquisitions - Aggregate Purchase Price and Allocation of Consideration (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Oct. 31, 2016	Oct. 31, 2015	Apr. 30, 2016	Sep. 01, 2016	Jan. 01, 2016	Apr. 30, 2015
Purchase price
Net cash consideration	$ 135,613	$ 859
Allocation of consideration for acquisitions
Goodwill	423,735		$ 386,306			$ 348,811
Adjustments/Reclassifications
Goodwill	(256)		(1,338)
2017 Acquisitions
Purchase price
Aggregate purchase price	144,079
Net cash consideration	139,171
Contingent consideration	4,908
Net sales increase from acquisitions	32,425
Operating income increase from acquisitions	1,615
Allocation of consideration for acquisitions
Cash and cash equivalents	1,554
Trade accounts and notes receivable	34,364
Inventories	13,125
Property and equipment	8,305
Other assets	489
Goodwill	37,685
Deferred tax liability	(5,898)
Liabilities assumed	(15,845)
Purchase price	144,079
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes	23,919
Other intangible assets expected to be deductible for U.S. federal income tax purposes	49,700
Goodwill nondeductible for U.S. federal income tax purposes	13,766
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	8,900
2017 Acquisitions | Customer relationships
Allocation of consideration for acquisitions
Definite lived intangible assets	61,400
Olympia Building Supplies, LLC/Redmill, Inc.
Business Acquisitions
Outstanding common stock purchased (as a percent)				100.00%
2016 Acquisitions
Purchase price
Aggregate purchase price			117,253
Net cash consideration			113,801
Contingent consideration			3,452
Net sales increase from acquisitions	112,329
Operating income increase from acquisitions	3,870
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	38,577
Deferred tax liability	(6,676)
Liabilities assumed	(13,797)
Purchase price	117,253
Adjustments/Reclassifications
Trade accounts and notes receivable	322
Other assets	1
Goodwill	(256)
Liabilities assumed	7
Purchase price	74
Decrease in total consideration paid	74
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
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%