Business Acquisitions (Details) $ in Thousands, shares in Millions, $ in Millions				12 Months Ended
	Jun. 01, 2018 USD ($) item shares	Jun. 01, 2018 CAD ($) shares	Jun. 07, 2017 USD ($)	Apr. 30, 2020 USD ($) segment	Apr. 30, 2019 USD ($)	Apr. 30, 2018 USD ($)	Nov. 01, 2019
Acquisitions
Loan repayment					$ 571,840	$ 477,616
Borrowings from the revolving credit facility				$ 880,698	981,148	513,878
Preliminary Purchase Price Allocation
Goodwill				$ 553,073	617,327	427,645
Number of reportable segments | segment				1
Goodwill and intangible assets
Goodwill expected to be deductible for U.S. federal income tax purposes				$ 2,100
Goodwill expected to be nondeductible for U.S. federal income tax purposes				3,200
Third Amendment
Acquisitions
Aggregate principal amount	$ 996,800
Third Amendment | LIBOR
Acquisitions
Margin added to variable rate (as a percent)	2.75%	2.75%
Variable rate floor (as a percent)	0.00%	0.00%
Term Loan Facility
Acquisitions
Loan repayment	$ 571,800		$ 477,600
Titan
Acquisitions
Number of locations | item	30
Number of provinces | item	5
Aggregate purchase price	$ 627,000	$ 800.0
Issuance of 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					5,573
Trade accounts and notes receivable					85,009
Inventories					60,272
Prepaid and other current assets					8,334
Property and equipment					37,263
Intangible assets					286,954
Goodwill					193,798
Accounts payable and accrued expenses					(41,803)
Contingent consideration					(12,039)
Deferred tax liability					(12,252)
Fair value of consideration transferred					611,109
Titan | Third Amendment
Acquisitions
Aggregate principal amount	$ 996,800
Titan | Third Amendment | LIBOR
Acquisitions
Margin added to variable rate (as a percent)	2.75%	2.75%
Variable rate floor (as a percent)	0.00%	0.00%
Titan | ABL facility
Acquisitions
Borrowings from the revolving credit facility	$ 143,000
Titan | Term Loan Facility
Acquisitions
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
Goodwill						9,358
2020 Acquisitions
Purchase price
Fair value of consideration transferred				25,100
Preliminary Purchase Price Allocation
Cash				713
Trade accounts and notes receivable				5,149
Inventories				7,194
Other current assets				72
Property and equipment				6,788
Goodwill				5,291
Liabilities assumed				(5,028)
Fair value of consideration transferred				25,075
2020 Acquisitions | Customer relationships
Preliminary Purchase Price Allocation
Intangible assets				4,252
2020 Acquisitions | Tradenames
Preliminary Purchase Price Allocation
Intangible assets				$ 644
Rigney Building Supplies Ltd
Acquisitions
Outstanding common stock purchased (as a percent)							100.00%
As previously reported | Titan
Preliminary Purchase Price Allocation
Cash	5,573
Trade accounts and notes receivable	84,039
Inventories	60,272
Prepaid and other current assets	8,334
Property and equipment	37,263
Intangible assets	289,423
Goodwill	196,524
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
Goodwill						9,358
Adjustment | Titan
Preliminary Purchase Price Allocation
Trade accounts and notes receivable					970
Intangible assets					(2,469)
Goodwill					(2,726)
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