Investments - Sales and Maturities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 27, 2018	Jan. 28, 2017	Jan. 27, 2018	Jan. 28, 2017
Sales of available-for-sale securities
Proceeds from sales	$ 5,580	$ 4,142	$ 17,109	$ 13,899
Gross realized gains	870	866	1,288	913
Gross realized losses	(284)	$ (366)	(512)	$ (404)
Fair value of available-for-sale securities by contractual maturity
Fair value, available-for-sale securities with contractual maturity within one year	12,800		12,800
Fair value, available-for-sale securities with contractual maturity within two to five years	22,500		22,500
Fair value, available-for-sale securities with contractual maturity within six to ten years	1,500		1,500
Fair value, available-for-sale securities with contractual maturity after ten years	$ 700		$ 700