Pension Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 27, 2018	Jan. 28, 2017	Jan. 27, 2018	Jan. 28, 2017
Net periodic pension costs
Service cost	$ 328	$ 320	$ 986	$ 958
Interest cost	1,147	1,170	3,441	3,510
Expected return on plan assets	(1,204)	(1,245)	(3,612)	(3,735)
Net amortization	780	764	2,340	2,292
Net periodic pension cost	$ 1,051	$ 1,009	$ 3,155	$ 3,025