Investments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 24, 2015	Oct. 25, 2014	Oct. 24, 2015	Oct. 25, 2014	Apr. 25, 2015
Summary of investments
Gross Unrealized Gains	$ 1,761		$ 1,761		$ 2,239
Gross Unrealized Losses	(141)		(141)		(114)
Fair Value	45,729		45,729		61,195
Sales of available-for-sale securities
Proceeds from sales	6,426	$ 7,405	19,409	$ 17,827
Gross realized gains	26	31	703	45
Gross realized losses	(33)	$ (22)	(539)	$ (30)
Fair value of available-for-sale securities by contractual maturity
Fair value, available-for-sale securities with contractual maturity within one year	10,700		10,700
Fair value, available-for-sale securities with contractual maturity within two to five years	24,400		24,400
Fair value, available-for-sale securities with contractual maturity within six to ten years	1,900		1,900
Fair value, available-for-sale securities with contractual maturity after ten years	100		100
Investments Used to Enhance Returns on Cash
Summary of investments
Fair Value	31,800		31,800		45,500
Other current assets
Investments
Available-for-sale investments	10,400		10,400		16,800
Trading securities					1,100
Other long-term assets
Investments
Available-for-sale investments	35,300		35,300		43,300
Equity securities
Summary of investments
Gross Unrealized Gains	1,609		1,609		2,014
Gross Unrealized Losses	(97)		(97)		(78)
Fair Value	8,261		8,261		9,251
Fixed income
Summary of investments
Gross Unrealized Gains	151		151		224
Gross Unrealized Losses	(23)		(23)		(14)
Fair Value	37,112		37,112		50,358
Mutual funds
Summary of investments
Fair Value					1,127
Other
Summary of investments
Gross Unrealized Gains	1		1		1
Gross Unrealized Losses	(21)		(21)		(22)
Fair Value	$ 356		$ 356		$ 459