Business Acquisitions (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015	Sep. 01, 2016	Jan. 01, 2016
Allocation of consideration for acquisitions
Goodwill	$ 423,644	$ 386,306	$ 348,811
Adjustments/Reclassifications
Goodwill	(390)	(1,338)
As previously reported
Adjustments/Reclassifications
Contingent consideration paid out		6,598	1,001
2017 Acquisitions
Purchase price
Aggregate purchase price	153,676
Cash consideration	148,705
Contingent consideration	4,971
Allocation of consideration for acquisitions
Cash and cash equivalents	1,558
Trade accounts and notes receivable	37,691
Inventories	16,504
Property and equipment	8,357
Other assets	657
Goodwill	37,728
Deferred tax liability	(6,011)
Liabilities assumed	(16,958)
Purchase price	153,676
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes	25,510
Other intangible assets expected to be deductible for U.S. federal income tax purposes	53,550
Goodwill nondeductible for U.S. federal income tax purposes	12,218
Other intangible assets nondeductible for U.S. federal income tax purposes	20,600
2017 Acquisitions | Selling, general and administrative expenses
Purchase price
Transaction costs	2,160
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
Olympia Building Supplies, LLC/Redmill, Inc.
Business Acquisitions
Outstanding common stock purchased (as a percent)				100.00%
2016 Acquisitions
Purchase price
Aggregate purchase price		117,181
Cash consideration		114,812
Contingent consideration		2,369
Allocation of consideration for acquisitions
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
Adjustments/Reclassifications
Cash and cash equivalents	(3)
Trade accounts and notes receivable	281
Inventories	160
Goodwill	(434)
Liabilities assumed	(1)
Purchase price	3
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes	13,357
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 | Selling, general and administrative expenses
Purchase price
Transaction costs		2,056
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		808
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%
2015 Acquisitions
Purchase price
Aggregate purchase price			72,208
Cash consideration			69,168
Contingent consideration			3,040
Allocation of consideration for acquisitions
Trade accounts and notes receivable		15,066
Inventories		8,760
Property and equipment		5,116
Other assets		76
Goodwill		20,337
Liabilities assumed		(11,247)
Purchase price		72,208
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes	20,337
Other intangible assets expected to be deductible for U.S. federal income tax purposes	$ 34,100
2015 Acquisitions | Selling, general and administrative expenses
Purchase price
Transaction costs		12	$ 945
2015 Acquisitions | Tradenames
Allocation of consideration for acquisitions
Definite lived intangible assets		3,260
2015 Acquisitions | Customer relationships
Allocation of consideration for acquisitions
Definite lived intangible assets		$ 30,840