Investments (Details) (USD $)	3 Months Ended
	Jul. 26, 2014	Jul. 27, 2013	Apr. 26, 2014
Summary of investments
Gross Unrealized Gains	$ 1,747,000		$ 1,413,000
Gross Unrealized Losses	(100,000)		(106,000)
Fair Value	60,770,000		60,938,000
Sales of available-for-sale securities
Proceeds from sales	10,422,000	8,558,000
Gross realized gains	14,000	13,000
Gross realized losses	(8,000)	(36,000)
Fair value of available-for-sale securities by contractual maturity
Fair value, available-for-sale securities with contractual maturity within one year	16,700,000
Fair value, available-for-sale securities with contractual maturity within two to five years	31,800,000
Fair value, available-for-sale securities with contractual maturity within six to ten years	1,900,000
Fair value, available-for-sale securities with contractual maturity after ten years	500,000
Investments Used to Enhance Returns on Cash [Member]
Summary of investments
Fair Value	45,200,000		44,700,000
Other Current Assets [Member]
Investment Holdings [Line Items]
Available-for-sale investments	16,700,000		15,900,000
Trading securities	1,000,000		1,800,000
Other Noncurrent Assets [Member]
Investment Holdings [Line Items]
Available-for-sale investments	43,100,000		43,200,000
Equity Securities [Member]
Summary of investments
Gross Unrealized Gains	1,565,000		1,246,000
Gross Unrealized Losses	(45,000)		(52,000)
Fair Value	8,574,000		8,216,000
Fixed Income Securities [Member]
Summary of investments
Gross Unrealized Gains	181,000		166,000
Gross Unrealized Losses	(40,000)		(44,000)
Fair Value	50,925,000		50,510,000
Asset-backed Securities [Member]
Summary of investments
Fair Value	1,017,000		1,787,000
Other Debt Obligations [Member]
Summary of investments
Gross Unrealized Gains	1,000		1,000
Gross Unrealized Losses	(15,000)		(10,000)
Fair Value	$ 254,000		$ 425,000