Investments (Details) (USD $)	3 Months Ended		6 Months Ended
	Oct. 25, 2014	Oct. 26, 2013	Oct. 25, 2014	Oct. 26, 2013	Apr. 26, 2014
Summary of investments
Gross Unrealized Gains	$ 1,631,000		$ 1,631,000		$ 1,413,000
Gross Unrealized Losses	(165,000)		(165,000)		(106,000)
Fair Value	59,913,000		59,913,000		60,938,000
Sales of available-for-sale securities
Proceeds from sales	7,405,000	9,636,000	17,827,000	18,196,000
Gross realized gains	31,000	32,000	45,000	45,000
Gross realized losses	(22,000)	(29,000)	(30,000)	(65,000)
Fair value of available-for-sale securities by contractual maturity
Fair value, available-for-sale securities with contractual maturity within one year	20,300,000		20,300,000
Fair value, available-for-sale securities with contractual maturity within two to five years	28,200,000		28,200,000
Fair value, available-for-sale securities with contractual maturity within six to ten years	1,900,000		1,900,000
Fair value, available-for-sale securities with contractual maturity after ten years	400,000		400,000
Investments Used to Enhance Returns on Cash
Summary of investments
Fair Value	45,200,000		45,200,000		44,700,000
Other Current Assets
Investment Holdings [Line Items]
Available-for-sale investments	20,300,000		20,300,000		15,900,000
Trading securities	500,000		500,000		1,800,000
Other Noncurrent Assets
Investment Holdings [Line Items]
Available-for-sale investments	39,100,000		39,100,000		43,200,000
Equity Securities
Summary of investments
Gross Unrealized Gains	1,436,000		1,436,000		1,246,000
Gross Unrealized Losses	(102,000)		(102,000)		(52,000)
Fair Value	8,410,000		8,410,000		8,216,000
Fixed Income Securities
Summary of investments
Gross Unrealized Gains	194,000		194,000		166,000
Gross Unrealized Losses	(45,000)		(45,000)		(44,000)
Fair Value	50,802,000		50,802,000		50,510,000
Asset-backed Securities
Summary of investments
Fair Value	493,000		493,000		1,787,000
Other Debt Obligations
Summary of investments
Gross Unrealized Gains	1,000		1,000		1,000
Gross Unrealized Losses	(18,000)		(18,000)		(10,000)
Fair Value	$ 208,000		$ 208,000		$ 425,000