Investments - Sales and Maturities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 28, 2017	Jan. 23, 2016	Jan. 28, 2017	Jan. 23, 2016
Sales of available-for-sale securities
Proceeds from sales	$ 4,142	$ 4,487	$ 13,899	$ 23,896
Gross realized gains	866	267	913	970
Gross realized losses	(366)	$ (18)	(404)	$ (557)
Fair value of available-for-sale securities by contractual maturity
Fair value, available-for-sale securities with contractual maturity within one year	14,100		14,100
Fair value, available-for-sale securities with contractual maturity within two to five years	18,300		18,300
Fair value, available-for-sale securities with contractual maturity within six to ten years	1,300		1,300
Fair value, available-for-sale securities with contractual maturity after ten years	$ 300		$ 300