Business Acquisitions (Details) $ in Thousands, shares in Millions, $ in Millions				12 Months Ended
	Jun. 01, 2018 CAD ($) shares	Jun. 01, 2018 USD ($) item shares	Jun. 07, 2017 USD ($)	Apr. 30, 2019 USD ($)	Apr. 30, 2018 USD ($)	Apr. 30, 2017 USD ($)	Sep. 01, 2016
Acquisition of Titan
Drew down				$ 981,148	$ 513,878	$ 1,013,365
Loan repayment						160,000
Preliminary Purchase Price Allocation
Goodwill				617,327	427,645	423,644
Third Amendment
Acquisition of Titan
Aggregate principal amount		$ 996,800
Third Amendment | LIBOR
Acquisition of Titan
Variable rate floor (as a percent)	0.00%	0.00%
Margin added to variable rate (as a percent)	2.75%	2.75%
First Lien Loan
Acquisition of Titan
Aggregate principal amount				550,000
Loan repayment		$ 571,800	$ 477,600
Titan
Acquisition of Titan
Number of locations | item		30
Number of provinces | item		5
Aggregate purchase price	$ 800.0	$ 627,000
Issuance of shares to current shareholders of Titan (in shares) | shares	1.1	1.1
Purchase price
Fair value of consideration transferred		$ 611,100
Cash		581,500
Issuance preferred stock to current shareholders of Titan		29,600
Preliminary Purchase Price Allocation
Cash and cash equivalents				5,573
Trade accounts and notes receivable				85,009
Inventories				60,272
Prepaid and other current assets				8,334
Property and equipment				37,263
Goodwill				193,798
Intangible assets		286,954		286,954
Accounts payable and accrued expenses				(41,803)
Contingent consideration				(12,039)
Deferred tax liability				(12,252)
Fair value of consideration transferred				611,109
Titan | Customer relationships
Preliminary Purchase Price Allocation
Intangible assets		246,954
Titan | Tradenames
Preliminary Purchase Price Allocation
Intangible assets		30,870
Titan | Third Amendment
Acquisition of Titan
Aggregate principal amount		$ 996,800
Titan | Third Amendment | LIBOR
Acquisition of Titan
Variable rate floor (as a percent)	0.00%	0.00%
Margin added to variable rate (as a percent)	2.75%	2.75%
Titan | ABL facility
Acquisition of Titan
Drew down		$ 143,000
Titan | First Lien Loan
Acquisition of Titan
Loan repayment		571,800
2018 Acquisitions
Purchase price
Fair value of consideration transferred					24,400
Preliminary Purchase Price Allocation
Trade accounts and notes receivable					4,872
Inventories					4,321
Property and equipment					1,081
Goodwill					4,153
Liabilities assumed					(1,951)
Fair value of consideration transferred					24,454
2018 Acquisitions | Tradenames
Preliminary Purchase Price Allocation
Intangible assets					1,000
2018 Acquisitions | Vendor agreement
Preliminary Purchase Price Allocation
Intangible assets					1,000
2018 Acquisitions | Other intangible assets
Preliminary Purchase Price Allocation
Intangible assets					620
2018 Acquisitions | Customer relationships
Preliminary Purchase Price Allocation
Intangible assets					9,358
2017 Acquisitions
Purchase price
Fair value of consideration transferred						154,000
Cash						148,700
Contingent consideration						5,300
Preliminary Purchase Price Allocation
Cash and cash equivalents						1,558
Trade accounts and notes receivable						37,628
Inventories						16,504
Other current assets						671
Property and equipment						8,357
Goodwill						37,584
Deferred tax liability						(6,011)
Liabilities assumed						(16,398)
Fair value of consideration transferred						154,043
Goodwill and intangible assets
Increase in total consideration paid					400
Goodwill expected to be deductible for U.S. federal income tax purposes					25,400
Other intangible assets expected to be deductible for U.S. federal income tax purposes					53,600
Goodwill expected to be nondeductible for U.S. federal income tax purposes					12,200
Other intangible assets expected to be nondeductible for U.S. federal income tax purposes					20,600
2017 Acquisitions | Customer relationships
Preliminary Purchase Price Allocation
Intangible assets						64,660
2017 Acquisitions | Tradenames
Preliminary Purchase Price Allocation
Intangible assets						9,490
Olympia Building Supplies, LLC/Redmill, Inc.
Acquisition of Titan
Outstanding common stock purchased (as a percent)							100.00%
As previously reported | Titan
Preliminary Purchase Price Allocation
Cash and cash equivalents		5,573
Trade accounts and notes receivable		84,039
Inventories		60,272
Prepaid and other current assets		8,334
Property and equipment		37,263
Goodwill		196,524
Intangible assets		289,423
Accounts payable and accrued expenses		(40,833)
Contingent consideration		(12,039)
Deferred tax liability		(14,337)
Fair value of consideration transferred		$ 614,219
As previously reported | 2018 Acquisitions
Preliminary Purchase Price Allocation
Trade accounts and notes receivable					4,872
Inventories					4,321
Property and equipment					1,081
Goodwill					4,145
Liabilities assumed					(1,951)
Fair value of consideration transferred					24,446
As previously reported | 2018 Acquisitions | Tradenames
Preliminary Purchase Price Allocation
Intangible assets					1,000
As previously reported | 2018 Acquisitions | Vendor agreement
Preliminary Purchase Price Allocation
Intangible assets					1,000
As previously reported | 2018 Acquisitions | Other intangible assets
Preliminary Purchase Price Allocation
Intangible assets					620
As previously reported | 2018 Acquisitions | Customer relationships
Preliminary Purchase Price Allocation
Intangible assets					9,358
As previously reported | 2017 Acquisitions
Preliminary Purchase Price Allocation
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)
Fair value of consideration transferred						153,676
As previously reported | 2017 Acquisitions | Customer relationships
Preliminary Purchase Price Allocation
Intangible assets						64,660
As previously reported | 2017 Acquisitions | Tradenames
Preliminary Purchase Price Allocation
Intangible assets						9,490
Adjustment | Titan
Preliminary Purchase Price Allocation
Trade accounts and notes receivable				970
Goodwill				(2,726)
Intangible assets				(2,469)
Accounts payable and accrued expenses				(970)
Deferred tax liability				2,085
Fair value of consideration transferred				(3,110)
Adjustment | 2018 Acquisitions
Preliminary Purchase Price Allocation
Goodwill				$ 8
Fair value of consideration transferred					$ 8
Adjustment | 2017 Acquisitions
Preliminary Purchase Price Allocation
Trade accounts and notes receivable						(63)
Other current assets						14
Goodwill						(144)
Liabilities assumed						560
Fair value of consideration transferred						$ 367