Acquisitions - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended
	Jan. 30, 2016	Aug. 01, 2015	Jan. 24, 2015	Jan. 30, 2016	Jan. 24, 2015	Apr. 25, 2015
Business Acquisition [Line Items]
Goodwill	$ 815,316,000			$ 815,316,000		$ 299,924,000
Revenues	1,400,853,000		$ 958,628,000	3,932,933,000	$ 2,875,804,000
Operating income	$ 95,729,000		$ 77,377,000	241,369,000	215,513,000
Tax impact of repatriation of foreign earnings					$ 12,300,000
Animal Health International [Member]
Business Acquisition [Line Items]
Business acquisition cash paid		$ 1,106,583,000
Identifiable net assets acquired		$ 588,213,000
Revision period after valuation completion for provisional amounts		1 year
Deferred tax asset		$ 2,569,000
Deferred tax liability		177,789,000
Goodwill		518,370,000
Goodwill recognized, deductible for income tax purpose		0
Deferred taxes recorded related to goodwill		0
Revenues				992,538,000
Operating income				22,196,000
Amortization expense related to identifiable intangible assets				20,080,000
Tax impact of repatriation of foreign earnings				$ 12,300,000
Animal Health International [Member] | Term Loan [Member]
Business Acquisition [Line Items]
Unsecured term loan		1,000,000,000
Animal Health International [Member] | Unsecured Revolving Line of Credit [Member]
Business Acquisition [Line Items]
Maximum borrowing capacity		$ 500,000,000